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.