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/LANES.md

@@ -472,7 +472,7 @@ curl -X POST http://localhost:4820/api/lanes/ensure \
 ```
 
 - If a lane already owns the `cwd` **for the requested `tool`**, returns that lane with `created: false` and HTTP `200`. `tool` (`claude` or `codex`, default `claude`) is part of the identity: adopting the same `cwd` with a different `tool` creates a second, sibling lane rather than adopting the existing one — one `cwd` can back at most one `claude` lane and one `codex` lane at a time. This applies to Repo/Adopt mode only; Worktree mode always gets its own directory.
-- If the `cwd` is a subdirectory of an existing lane with that same tool, returns the parent lane that owns it.
+- If the `cwd` is a subdirectory of an existing lane with that same tool, returns the parent lane that owns it. Pass `"exact": true` to skip that: only a lane on this very directory is returned, and a nested one is created as its own lane — the Add-lane modal does this, and shows which lane already owns the folder when the answer is `created: false`.
 - If no lane owns the `(cwd, tool)` pair, creates an **adopted** lane and returns it with `created: true` and HTTP `201`.
 - A relative or missing `cwd` returns `400 EBADCWD`.