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/settings.json

@@ -495,5 +495,19 @@
     "testing": "Testing…",
     "testOk": "The endpoint answered — {{model}} is reachable with this key.",
     "testFailed": "No answer ({{status}}): {{message}}"
+  },
+  "appearance": {
+    "title": "Appearance",
+    "description": "Colour mode and theme. Saved in this browser only.",
+    "mode": "Mode",
+    "modes": {
+      "dark": "Dark",
+      "light": "Light",
+      "system": "System"
+    },
+    "darkThemes": "Dark themes",
+    "lightThemes": "Light themes",
+    "active": "· in use now",
+    "default": "Default"
   }
 }