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/__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)/);
+  });
+});