Add bundled default sound pack with auto-copy functionality

Features:
- Complete default sound pack with 14 OGG audio files covering all Bifrost events
- Auto-copy bundled pack to user data directory on first run (XDG compliant)
- Graceful fallback if bundled pack is missing or copy fails
- No network dependency for immediate audio feedback

Sound Pack Contents:
- Core sounds: startup, shutdown, success, error
- Communication: private_message, mention, boost, reply, post_sent
- Interface: timeline_update, notification
- Interaction: autocomplete, autocomplete_end

Technical Implementation:
- Enhanced SoundManager.create_default_pack() to copy from bundled location
- Smart path detection for bundled pack across different installation scenarios
- Robust error handling with minimal fallback pack.json creation
- Total pack size: ~260KB (suitable for regular Git without LFS)

User Experience:
- Immediate audio feedback from first application launch
- No setup or configuration required
- Sounds install to proper XDG data directory (~/.local/share/bifrost/sounds/)
- Users can still customize or replace the default pack

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

sounds/default/pack.json

@@ -0,0 +1,21 @@
+{
+  "name": "Default",
+  "description": "Clean, accessible default sounds for Bifrost",
+  "author": "Bifrost Development Team",
+  "version": "1.0",
+  "sounds": {
+    "private_message": "private_message.ogg",
+    "mention": "mention.ogg",
+    "boost": "boost.ogg",
+    "reply": "reply.ogg",
+    "post_sent": "post_sent.ogg",
+    "timeline_update": "timeline_update.ogg",
+    "notification": "notification.ogg",
+    "autocomplete": "autocomplete.ogg",
+    "autocomplete_end": "autocomplete_end.ogg",
+    "startup": "startup.ogg",
+    "shutdown": "shutdown.ogg",
+    "success": "success.ogg",
+    "error": "error.ogg"
+  }
+}