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__/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", () => {