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

server/openapi-extra/lanes.js

@@ -60,6 +60,11 @@ const schemas = {
           "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: {
@@ -111,7 +116,7 @@ const 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.",
+        "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,