Add dark color scheme, backlight timeout, and L/R swap feature

- Invert UI colors: black background with white text, yellow highlights
- Add 30-second backlight timeout with auto-wake on button press
- Add Swap L/R menu option with checkbox to swap audio channels
- Update volume page styling (yellow bar, full-screen container)

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

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

main/system.h

@@ -55,6 +55,9 @@ typedef struct SystemState_s
     // Charge status
     bool isCharging;
 
+    // Swap L/R audio channels
+    bool swapLR;
+
 } SystemState_t;
 
 
@@ -90,6 +93,10 @@ float system_getAngle(void);
 void system_setChargeStatus(bool charging);
 bool system_getChargeStatus(void);
 
+void system_setSwapLR(bool swap);
+bool system_getSwapLR(void);
+void system_toggleSwapLR(void);
+
 void system_setZeroAngle(void);
 void system_clearZeroAngle(void);
 float system_getZeroAngle(void);