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/index.html

@@ -1,6 +1,24 @@
 <!doctype html>
 <html lang="en" class="dark">
   <head>
+    <!-- Apply the saved appearance before first paint (same keys as
+         src/hooks/useTheme.ts, which re-applies and validates on mount). -->
+    <script>
+      try {
+        var m = localStorage.getItem("theme");
+        var d =
+          m === "light"
+            ? false
+            : m === "system"
+              ? !(window.matchMedia && !matchMedia("(prefers-color-scheme: dark)").matches)
+              : true;
+        var r = document.documentElement;
+        var t = localStorage.getItem("agenthub.theme." + (d ? "dark" : "light"));
+        r.classList.toggle("dark", d);
+        r.setAttribute("data-mode", d ? "dark" : "light");
+        if (t && t !== "default") r.setAttribute("data-theme", t);
+      } catch (e) {}
+    </script>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <link rel="icon" type="image/x-icon" href="/favicon.ico" />