Add per-notification custom sounds (supersedes #249)

[Finesssee]

Supersedes

• Add per-notification custom sounds #249 (author @yozu). Contains that branch's commits unchanged, plus review follow-ups merged from latest main.

Summary

Per-notification-event custom WAV sounds: 7 embedded built-in sounds (48 kHz stereo PCM), per-event choose/preview/clear, Windows system-sound mapping preserved as default, serialized playback worker, toast audio silenced, save-time WAV validation (RIFF chunk walk), data-driven settings UI.

Review fixes on top of #249

• Removed the dead sound_volume chain end-to-end (settings, bridge types, locale keys in all 7 catalogs, test mocks) — the field was never consumed by playback.
• WAV validation happens at save only (play-time re-read dropped; failures surface as PlaybackFailed).
• Replaced the as NotificationSoundTheme cast with a typed guard.
• Includes the locale-catalog test update Russian requires (was red on main after Add Russian localization #248).

Validation

• CUA proof on a fresh Windows debug build (proof-249/*.png): notifications surface shows 7 event rows + default sound set select; preview switches to "Playing…" and serializes (all 7 test buttons report IsEnabled=false during playback); restores after; theme switches Windows↔CodexBar both ways.
• local-check.ps1 exit 0 on the branch; 857 rust + 328 tauri + 215 frontend tests; clippy -D warnings both crates; check-locale 732 keys.

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