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

README.md

@@ -64,6 +64,10 @@ Internal build — all rights reserved.
   `settings.json` stay read-only: they race with the live CLI.
 - **Updates.** Release-based update detection with an in-UI **Update all**
   (marketplaces → every plugin → AgentHub last → one session reload).
+- **Appearance.** Settings → Appearance picks Dark / Light / System (follows the
+  OS live) and a theme per mode — dark: Default, Darcula, Apple Dark; light:
+  Default, Apple Light, IntelliJ Light, Canvas. Each mode remembers its own
+  theme, so the sidebar's quick toggle swaps between the two. Per-browser only.
 - **A CLI for all of it** (`agenthub`, plus `agenthub repl`), a PWA install, and
   a UI in English, Vietnamese, Chinese and Korean.