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/README.md

@@ -52,7 +52,7 @@ The client is a single-page application (SPA) built with modern web technologies
 - **React 18.3** - Component-based UI with hooks and concurrent features
 - **TypeScript 5.7** - Full type safety across components, utilities, and API contracts
 - **Vite 6.1** - Lightning-fast HMR during development, optimized production builds
-- **Tailwind CSS 3.4** - Utility-first CSS framework; colors are CSS-variable-backed tokens (`darkMode: "class"`) so a Dark/Light/System switch (next to the language switcher in the sidebar, and in Settings → Appearance; `useTheme` hook) re-themes the whole app by flipping one class. A **named theme** per mode is one more attribute: `data-theme` on `<html>` (absent for `default`) selects a CSS-variable block in `index.css` — `.dark[data-theme="darcula|apple-dark"]` and `[data-mode="light"][data-theme="apple-light|intellij-light|canvas"]` — that overrides only the tokens it changes. `--tint-base` is `var(--surface-3)` and light `--syn-plain` is `var(--fg-primary)`, so a theme never restates them. A new theme is a block there plus an id in `THEMES` (`hooks/useTheme.ts`) and a label in `components/settings/AppearanceSettings.tsx`; its Settings preview needs no hex, because the swatch re-scopes the same variables on its own element. Storage: `theme` = `dark|light|system`, `agenthub.theme.dark` / `agenthub.theme.light` = theme ids (unknown ⇒ `default`); state is read through `useSyncExternalStore`, never cached in the module, so every consumer and every tab agree. An inline `<script>` at the top of `client/index.html` applies the same keys before first paint (no flash of the stock dark palette); it does not validate ids — the hook does, on mount
+- **Tailwind CSS 3.4** - Utility-first CSS framework; colors are CSS-variable-backed tokens (`darkMode: "class"`) so a Dark/Light/System switch (next to the language switcher in the sidebar, and in Settings → Appearance; `useTheme` hook) re-themes the whole app by flipping one class. A **named theme** per mode is one more attribute: `data-theme` on `<html>` (absent for `default`) selects a CSS-variable block in `index.css` — `.dark[data-theme="darcula|apple-dark|orca-dark"]` and `[data-mode="light"][data-theme="apple-light|intellij-light|canvas|orca-light"]` — that overrides only the tokens it changes. `--tint-base` is `var(--surface-3)` and light `--syn-plain` is `var(--fg-primary)`, so a theme never restates them. A new theme is a block there plus an id in `THEMES` (`hooks/useTheme.ts`) and a label in `components/settings/AppearanceSettings.tsx`; its Settings preview needs no hex, because the swatch re-scopes the same variables on its own element. Storage: `theme` = `dark|light|system`, `agenthub.theme.dark` / `agenthub.theme.light` = theme ids (unknown ⇒ `default`); state is read through `useSyncExternalStore`, never cached in the module, so every consumer and every tab agree. An inline `<script>` at the top of `client/index.html` applies the same keys before first paint (no flash of the stock dark palette); it does not validate ids — the hook does, on mount
 
   **The surface ramp means elevation, and it runs the same direction in both themes: a higher index is LIGHTER in dark and LIGHTER in light.** So in light mode the page is the *darkest* step (`surface-0`, `#e8e5da`) and a `.card` is brighter than the page it sits on (`surface-3`, `#f5f3ec`), which is also what makes every `hover:bg-surface-N` lift rather than dim. Light mode originally used Radix slate in its native direction — index up meant darker — which inverted the whole language: a near-white page (the glare complaint) under grey cards that read as recessed. Never "fix" a light-mode colour by swapping which step a component uses; fix the value of the step.
 
@@ -221,7 +221,7 @@ client/
 │   ├── hooks/
 │   │   ├── useWebSocket.ts      # Auto-reconnecting WebSocket hook
 │   │   ├── useLanes.ts          # ONE shared lane list (GET /api/lanes + `lane_update` frames, refetch on reconnect) for the sidebar tree, status bar and Workspace page
-│   │   ├── useNotifications.ts  # Browser push notification triggers
+│   │   ├── useNotifications.ts  # Browser push notification triggers; a shown one plays its type's sound (`notifPrefs.sound`/`soundByType`, `playSound` in lib/notify.ts, WAVs in public/sounds/ generated by scripts/gen-notification-sounds.js)
 │   │   └── useTheme.ts          # Dark/light/system mode + per-mode named theme: `dark` class, `data-mode`, `data-theme` on <html>, persisted to localStorage
 │   │
 │   ├── i18n/               # Internationalization (en / vi)