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/hooks/useTheme.ts

@@ -18,8 +18,8 @@ export type ThemeMode = Theme | "system";
 
 /** Named themes per mode, in picker order. `default` is the stock palette. */
 export const THEMES = {
-  dark: ["default", "darcula", "apple-dark"],
-  light: ["default", "apple-light", "intellij-light", "canvas"],
+  dark: ["default", "darcula", "apple-dark", "orca-dark"],
+  light: ["default", "apple-light", "intellij-light", "canvas", "orca-light"],
 } as const satisfies Record<Theme, readonly string[]>;
 
 const MODE_KEY = "theme";