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.

client/src/components/settings/AppearanceSettings.tsx

@@ -19,6 +19,8 @@ const THEME_LABELS: Record<string, string> = {
   "apple-light": "Apple Light",
   "intellij-light": "IntelliJ Light",
   canvas: "Canvas",
+  "orca-dark": "Orca Dark",
+  "orca-light": "Orca Light",
 };
 
 const MODES: { mode: ThemeMode; Icon: typeof Moon }[] = [