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/i18n/locales/en/lanes.json

@@ -34,6 +34,7 @@
   "addLaneBranchPlaceholder": "feat/my-feature",
   "addLaneNoBranches": "This repo has no commits yet — the worktree will start empty.",
   "addLaneNoFreeBranches": "Every branch in this repo is already checked out in a worktree. Create a new branch instead.",
+  "addLaneExists": "This folder already has a lane: #{{id}} {{title}}. It is selected; pick another folder to add a new one.",
   "addLaneNotARepo": "Not a git repository (or no read access) yet.",
   "addLaneProvisionFailed": "Worktree provisioning failed or timed out — the lane exists but auto-setup was skipped. Check the lane's git facts, then run agents/mcp setup manually.",
   "addLaneRepoHint": "An existing git repo. The dashboard creates a new worktree for the lane, not a folder you pick.",