feat(ui): Appearance settings — dark/light/system mode and named themes; fix Add lane silently doing nothing (#42)

- Settings → Appearance: mode Dark / Light / System (follows the OS live) and
  a named theme remembered per mode — dark: Default, Darcula, Apple Dark;
  light: Default, Apple Light, IntelliJ Light, Canvas. Each theme is a
  CSS-variable override block keyed by `data-theme` on <html>; the sidebar
  quick toggle gains System and swaps between the two remembered themes.
- An inline boot script in index.html applies the stored mode and theme
  before first paint, so a reload no longer flashes the default colours.
- Diff +/- rows are easier to tell apart on every theme: a /20 status tint,
  a 3px status bar and a bold coloured marker; the changed-span emphasis no
  longer vanishes on light themes.
- #42: the Add-lane modal (Repo mode / "Current branch") asks
  POST /api/lanes/ensure with `exact: true`. Without it a folder nested inside
  an existing lane resolved to that parent lane with created:false, so the
  modal closed and no new card appeared. When the folder really already has a
  lane, the modal now says which one instead of closing.

Closes #42

client/src/components/lanes/AddLaneModal.tsx

@@ -246,6 +246,8 @@ export function AddLaneModal({
           cwd: repo,
           title: name || undefined,
           tool,
+          // THIS directory, not a lane on a parent of it (#42).
+          exact: true,
         });
         // Report the lane the INSTANT it exists server-side. Everything after
         // this is post-creation setup that can fail on its own, and letting one
@@ -253,6 +255,18 @@ export function AddLaneModal({
         // the list until some unrelated refetch happened to pick it up.
         // Worktree mode below already works this way.
         onAdded(result.lane);
+        // Nothing was created: closing now reads as success while no new card
+        // appears (#42). Say which lane already owns the folder instead.
+        if (!result.created) {
+          setError(
+            t("addLaneExists", {
+              id: result.lane.id,
+              title: result.lane.title || result.lane.cwd,
+            })
+          );
+          setBusy(false);
+          return;
+        }
 
         // A session that booted on the vendor's API would keep talking to it
         // for its whole life, so the router is applied before anything runs.

client/src/components/lanes/LaneDiffView.tsx

@@ -45,7 +45,7 @@ function Code({ text, lang, emph }: { text: string; lang: string; emph?: [number
     return (
       <>
         <Code text={text.slice(0, from)} lang={lang} />
-        <span className="rounded-sm bg-white/15">
+        <span className="rounded-sm bg-fg-primary/15">
           <Code text={text.slice(from, to)} lang={lang} />
         </span>
         <Code text={text.slice(to)} lang={lang} />
@@ -120,12 +120,21 @@ function Row({ tint, children }: { tint: string; children: React.ReactNode }) {
   return <div className={`flex w-max min-w-full ${tint}`}>{children}</div>;
 }
 
+/** A /20 fill plus a 3px status bar at the row's left edge, so +/- rows still
+ *  separate from context at a glance on every theme (the /10 fill alone did not). */
 function rowTint(type: LaneDiffLine["type"] | null) {
-  if (type === "add") return "bg-status-success/10";
-  if (type === "del") return "bg-status-danger/10";
+  if (type === "add")
+    return "bg-status-success/20 shadow-[inset_3px_0_0_rgb(var(--status-success))]";
+  if (type === "del") return "bg-status-danger/20 shadow-[inset_3px_0_0_rgb(var(--status-danger))]";
   return "";
 }
 
+function markerClass(type: LaneDiffLine["type"]) {
+  if (type === "add") return "font-bold text-status-success";
+  if (type === "del") return "font-bold text-status-danger";
+  return "text-fg-muted";
+}
+
 function HunkHeader({ hunk }: { hunk: LaneDiffHunk }) {
   const at = hunk.header.split("@@")[1];
   return (
@@ -153,7 +162,7 @@ function UnifiedHunk({ hunk, lang }: { hunk: LaneDiffHunk; lang: string }) {
           <Gutter n={line.oldLine} />
           <Gutter n={line.newLine} />
           <span className="whitespace-pre px-2">
-            <span className="select-none text-fg-muted">
+            <span className={`select-none ${markerClass(line.type)}`}>
               {line.type === "add" ? "+" : line.type === "del" ? "-" : " "}
             </span>
             <Code text={line.text} lang={lang} emph={emph.get(i)} />

client/src/components/lanes/__tests__/AddLaneModal.test.tsx

@@ -503,6 +503,7 @@ describe("AddLaneModal — repo mode (adopt)", () => {
         cwd: "/Users/tester/projects/repo",
         title: "main repo",
         tool: "claude",
+        exact: true,
       })
     );
     expect(api.lanes.worktree).not.toHaveBeenCalled();
@@ -511,6 +512,28 @@ describe("AddLaneModal — repo mode (adopt)", () => {
     // close right away like the old adopt flow did.
     expect(onClose).toHaveBeenCalled();
   });
+
+  it("says the folder already has a lane instead of closing silently (#42)", async () => {
+    vi.mocked(api.lanes.ensure).mockResolvedValue({
+      lane: laneFixture({ id: 3, kind: "adopted", status: "idle", title: "SG - Services" }),
+      created: false,
+    });
+    const onAdded = vi.fn();
+    const onClose = vi.fn();
+    renderModal({ onAdded, onClose });
+    const user = userEvent.setup();
+
+    await user.click(screen.getByRole("button", { name: "Repo" }));
+    const dirField = screen.getByLabelText("Directory");
+    await focusField(user, dirField);
+    await user.type(dirField, "/Users/tester/projects/repo");
+    await user.click(screen.getByRole("button", { name: "Add lane" }));
+
+    expect(await screen.findByText(/already has a lane.*#3.*SG - Services/)).toBeInTheDocument();
+    // The existing lane is selected so the user sees it, but nothing claims a new one.
+    expect(onAdded).toHaveBeenCalledWith(expect.objectContaining({ id: 3 }));
+    expect(onClose).not.toHaveBeenCalled();
+  });
 });
 
 describe("AddLaneModal — folder browse", () => {

client/src/components/lanes/__tests__/LaneDiffView.test.tsx

@@ -72,3 +72,89 @@ describe("LaneDiffView — side by side", () => {
     expect(screen.getByTestId("lane-diff-view").className).toContain("overflow-auto");
   });
 });
+
+function fileWith(lines: NonNullable<LaneChangeFile["hunks"]>[number]["lines"]): LaneChangeFile {
+  return { ...FILE, hunks: [{ ...FILE.hunks![0]!, lines }] };
+}
+
+const ROWS = fileWith([
+  { type: "ctx", oldLine: 1, newLine: 1, text: "keep()" },
+  { type: "del", oldLine: 2, newLine: null, text: "const x = 1;" },
+  { type: "add", oldLine: null, newLine: 2, text: "const x = 2;" },
+]);
+
+/** The unified row whose text contains `needle`. */
+function rowOf(container: HTMLElement, needle: string): HTMLElement {
+  const row = [...container.querySelectorAll<HTMLElement>("div.flex.w-max")].find((r) =>
+    r.textContent?.includes(needle)
+  );
+  if (!row) throw new Error(`no row for ${needle}`);
+  return row;
+}
+
+describe("LaneDiffView — row styling", () => {
+  it("tints add/del rows with their status fill and left bar; context stays plain", () => {
+    const { container } = render(<LaneDiffView file={ROWS} layout="unified" />);
+    const add = rowOf(container, "x = 2");
+    const del = rowOf(container, "x = 1");
+    const ctx = rowOf(container, "keep()");
+    expect(add.className).toContain("bg-status-success/20");
+    expect(add.className).toContain("--status-success");
+    expect(del.className).toContain("bg-status-danger/20");
+    expect(del.className).toContain("--status-danger");
+    expect(ctx.className).not.toMatch(/status-(success|danger)/);
+  });
+
+  it("colours the +/- marker bold in its status colour, context marker muted", () => {
+    const { container } = render(<LaneDiffView file={ROWS} layout="unified" />);
+    const marker = (row: HTMLElement) =>
+      row.querySelector<HTMLElement>("span.select-none:not(.w-10)")!;
+    const plus = marker(rowOf(container, "x = 2"));
+    const minus = marker(rowOf(container, "x = 1"));
+    const ctx = marker(rowOf(container, "keep()"));
+    expect(plus.textContent).toBe("+");
+    expect(plus.className).toContain("font-bold");
+    expect(plus.className).toContain("text-status-success");
+    expect(minus.textContent).toBe("-");
+    expect(minus.className).toContain("text-status-danger");
+    expect(ctx.className).toContain("text-fg-muted");
+    expect(ctx.className).not.toContain("font-bold");
+  });
+
+  it("emphasises only the changed span of a paired del/add", () => {
+    const { container } = render(<LaneDiffView file={ROWS} layout="unified" />);
+    const emph = [...container.querySelectorAll<HTMLElement>("span.bg-fg-primary\\/15")].map(
+      (e) => e.textContent
+    );
+    expect(emph).toEqual(["1", "2"]);
+  });
+
+  it("does not emphasise a line that changed completely", () => {
+    const { container } = render(
+      <LaneDiffView
+        file={fileWith([
+          { type: "del", oldLine: 1, newLine: null, text: "abc" },
+          { type: "add", oldLine: null, newLine: 1, text: "xyz" },
+        ])}
+        layout="unified"
+      />
+    );
+    expect(container.querySelector("span.bg-fg-primary\\/15")).toBeNull();
+  });
+
+  it("split view tints each side, and a filler row opposite an unpaired add stays plain", () => {
+    render(
+      <LaneDiffView
+        file={fileWith([{ type: "add", oldLine: null, newLine: 1, text: "added()" }])}
+        layout="split"
+      />
+    );
+    const rows = (id: string) => [
+      ...screen.getByTestId(id).querySelectorAll<HTMLElement>("div.flex.w-max"),
+    ];
+    const [right] = rows("lane-diff-split-right");
+    const [left] = rows("lane-diff-split-left");
+    expect(right!.className).toContain("bg-status-success/20");
+    expect(left!.className).not.toMatch(/status-(success|danger)/);
+  });
+});