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/lib/api.ts

@@ -2297,7 +2297,14 @@ export const api = {
      *   location isn't tied to a specific project (e.g. Sagi's mascot lane).
      * @returns `{ lane, created }` — the lane (newly created or existing) and whether it was created.
      */
-    ensure: (body: { cwd?: string; title?: string; pipeline?: string; tool?: LaneTool }) =>
+    ensure: (body: {
+      cwd?: string;
+      title?: string;
+      pipeline?: string;
+      tool?: LaneTool;
+      /** Only a lane on THIS directory counts, not one on a parent of it. */
+      exact?: boolean;
+    }) =>
       request<{ lane: Lane; created: boolean }>("/lanes/ensure", {
         method: "POST",
         body: JSON.stringify(body),