feat(ui): add bundled custom notification sounds

Adds three embedded WAV files as additional notification sound choices
(ids 17-19) alongside the existing 16 game sounds. Playback via NAudio
WaveOutEvent/WinMM, which works correctly on Wine/Linux.

docs/CREDITS.md

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+# Third-Party Asset Credits
+
+## Bundled Notification Sounds
+
+- **notification-1.wav, notification-2.wav, notification-3.wav**
+  Creator: Universfield
+  Source: [Pixabay](https://pixabay.com/users/universfield-28281460/)
+  License: Pixabay Content License (royalty-free, commercial use permitted)
+
+## Branding
+
+- **Fox banner image** (`fox-banner.png`)
+  Creator: InklyTattooDesigns (Etsy)
+  License: Royalty-free for commercial and logo use
+
+## Libraries
+
+- **NAudio** - Copyright (c) Mark Heath
+  License: MIT
+  Source: <https://github.com/naudio/NAudio>