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

openapi.yaml

@@ -13,7 +13,7 @@ externalDocs:
   description: Project documentation
   url: https://git.smartgift.io.vn/Smartgift-AI/AgentHub#readme
 servers:
-  - url: http://localhost:4820
+  - url: http://localhost:4830
     description: Local dashboard server (default)
   - url: http://127.0.0.1:4820
     description: Local loopback endpoint used by hook-handler
@@ -3906,6 +3906,9 @@ components:
           type: string
           description: Title for the lane if one has to be created; ignored when one already exists.
           example: App package
+        exact:
+          type: boolean
+          description: Only a lane on this very directory counts; a parent directory's lane is not returned, so a nested directory gets its own lane.
     LaneDestructiveActionRequest:
       type: object
       required:
@@ -10633,7 +10636,7 @@ paths:
       tags:
         - Lanes
       summary: Find or adopt the lane owning a working directory
-      description: 'Idempotent: returns the lane whose cwd is an exact match or the longest path-boundary parent of `cwd` with `created: false` (200), otherwise creates an `adopted` lane and returns it with `created: true` (201). The Workspace page opens on a directory rather than a lane id, so this is how it gets exactly one lane for that directory. Concurrent calls for the same path resolve to ONE lane: the `lanes.cwd` UNIQUE constraint decides, and the loser re-reads and returns the winner''s lane.'
+      description: 'Idempotent: returns the lane whose cwd is an exact match or the longest path-boundary parent of `cwd` with `created: false` (200) — with `exact: true` only an exact match counts, so a directory nested inside another lane gets a lane of its own (the Add-lane modal sends it) — otherwise creates an `adopted` lane and returns it with `created: true` (201). The Workspace page opens on a directory rather than a lane id, so this is how it gets exactly one lane for that directory. Concurrent calls for the same path resolve to ONE lane: the `lanes.cwd` UNIQUE constraint decides, and the loser re-reads and returns the winner''s lane.'
       operationId: ensureLane
       requestBody:
         required: true