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/lib/notifPrefs.ts

@@ -3,12 +3,26 @@
  * the defaults, and the `localStorage` read/write. Settings (which edits them)
  * and `useNotifications` (which obeys them) both import from here; they used to
  * carry private copies, so adding a notification type meant editing the same
- * literal twice and a miss went unnoticed until a toggle did nothing.
+ * literal twice and a miss went unnoticed until a toggle did nothing. A
+ * shown notification also plays a sound: `sound` is the default, and
+ * `soundByType` may override it per notification type (`soundFor`).
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
 export const NOTIF_KEY = "agent-monitor-notifications";
 
+export const NOTIF_SOUNDS = ["none", "chime", "pop", "alert"] as const;
+export type NotifSound = (typeof NOTIF_SOUNDS)[number];
+
+/** The per-type flags — every key a notification is gated on. */
+export type NotifType =
+  | "onNewSession"
+  | "onSessionError"
+  | "onAwaitingInput"
+  | "onTurnComplete"
+  | "onSessionComplete"
+  | "onSubagentSpawn";
+
 export interface NotifPrefs {
   /** Master switch; when false no notification fires, whatever the flags say. */
   enabled: boolean;
@@ -21,6 +35,9 @@ export interface NotifPrefs {
   /** The whole session closed (`SessionEnd`), not just one turn. */
   onSessionComplete: boolean;
   onSubagentSpawn: boolean;
+  /** The sound a shown notification plays, unless its type overrides it. */
+  sound: NotifSound;
+  soundByType?: Partial<Record<NotifType, NotifSound>>;
 }
 
 // The two "Claude is waiting for you" types default ON: they are the reason
@@ -34,8 +51,14 @@ export const defaultNotifPrefs: NotifPrefs = {
   onTurnComplete: true,
   onSessionComplete: false,
   onSubagentSpawn: false,
+  sound: "none",
 };
 
+/** The sound for one notification type: its override, else the default. */
+export function soundFor(prefs: NotifPrefs, type: NotifType): NotifSound {
+  return prefs.soundByType?.[type] ?? prefs.sound;
+}
+
 /** Reads prefs, merged over the defaults so a stored blob written by an older
  * build (missing the newer flags) still yields a complete object. */
 export function loadNotifPrefs(): NotifPrefs {