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/routes/lanes.js

@@ -318,7 +318,15 @@ router.post("/ensure", sameOriginGuard, (req, res) => {
   // already own this (cwd, tool) pair" is the real question, so a request for
   // a different tool than an existing lane at this cwd falls through to
   // creating that tool's own lane instead of adopting the wrong one.
-  const owner = lanesLib.resolveLaneByCwd(cwd, body.tool || "claude");
+  //
+  // `exact: true` is the Add-lane modal's question — "is THIS directory a
+  // lane" — so a lane on a PARENT directory does not answer it (#42: the
+  // modal closed on the parent lane and no new card ever appeared). Without
+  // it, a nested cwd still resolves to its owner, which is what the CLI and
+  // other callers adopting "the lane I am inside" rely on.
+  const found = lanesLib.resolveLaneByCwd(cwd, body.tool || "claude");
+  const strip = (p) => p.replace(/\/+$/, "");
+  const owner = found && (body.exact !== true || strip(found.cwd) === strip(cwd)) ? found : null;
   if (owner) {
     // First-write wins on the title: an ensure-with-title on an existing lane
     // is how callers like Sagi adopt the lane, so don't clobber a real title