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

docs/API.md

@@ -145,7 +145,10 @@ path. Existing cwd values still pass through unchanged. `pipeline` is
 optional and, like `title`, applies **only when a lane is actually created**
 — an existing lane is returned untouched. Omitted, the lane gets `default`.
 `POST /api/lanes/worktree` and `POST /api/lanes` take the same optional
-field. An id no template defines returns `400 EBADPIPELINE`.
+field. `exact: true` makes only a lane on this very directory count: without
+it a `cwd` nested inside another lane returns that parent lane (`created:
+false`); with it the nested directory gets a lane of its own. The Add-lane
+modal sends it, and says so when the directory already has a lane. An id no template defines returns `400 EBADPIPELINE`.
 
 `tool` (`"claude"` or `"codex"`, default `"claude"`) is the CLI this lane's
 runs will spawn. Immutable once the lane is created; an attempt to update it on