Add separate volume controls for greeting, button, and beep sounds

Added individual volume control for each sound type:
- Greeting volume (for welcome messages)
- Button sound volume (for extra button during recording)
- Beep volume (for recording start indicator)

Backend changes:
- Added volume_greeting, volume_button, volume_beep to config
- New getter/setter methods for each volume type
- API endpoints: /api/volume/greeting, /api/volume/button, /api/volume/beep
- Updated play_sound_file() with volume_override parameter
- Greeting, button, and beep playback now use their specific volumes

Frontend changes (template v1.7.0):
- Replaced single volume slider with three separate sliders
- Clean UI with labeled controls for each sound type
- Helper function setupVolumeSlider() for DRY code
- Real-time updates with debouncing (300ms)
- Visual feedback with gradient background

Config changes:
- Added volume_greeting: 70 (default)
- Added volume_button: 70 (default)
- Added volume_beep: 70 (default)
- Backward compatible with existing configs

This allows independent control of each sound's loudness,
useful when greeting needs to be louder than beep, etc.

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

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

config.example.json

@@ -38,6 +38,9 @@
     "beep_sound": "beep.wav",
     "beep_enabled": true,
     "greeting_delay_seconds": 0,
-    "volume": 70
+    "volume": 70,
+    "volume_greeting": 70,
+    "volume_button": 70,
+    "volume_beep": 70
   }
 }