feat(ui): Orca Dark / Orca Light themes and notification sounds

Two optional named themes from Orca's neutral palette join the Appearance
picker (Orca Dark keeps a blue accent so white-on-accent buttons stay
legible; Orca Light's status colours are darkened to clear 4.5:1 on their
chips). A shown notification can play a sound — None (default), Chime, Pop
or Alert — chosen globally and per notification type in Settings →
Notifications; the WAVs are generated by scripts/gen-notification-sounds.js.

README.md

@@ -69,9 +69,13 @@ Internal build — all rights reserved.
 - **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.
+  OS live) and a theme per mode — dark: Default, Darcula, Apple Dark, Orca Dark;
+  light: Default, Apple Light, IntelliJ Light, Canvas, Orca Light. Each mode
+  remembers its own theme, so the sidebar's quick toggle swaps between the two.
+  Per-browser only.
+- **Notification sounds.** Settings → Notifications picks a sound (None, Chime,
+  Pop, Alert — off by default) played with every notification shown, and each
+  notification type can pick its own.
 - **A CLI for all of it** (`agenthub`, plus `agenthub repl`), a PWA install, and
   a UI in English, Vietnamese, Chinese and Korean.