refactor: remove unused Vite configuration files and update documentation

Remove obsolete Vite configuration files that are no longer used by the build system
and update BUILDING.md to accurately reflect the current configuration structure.

**Removed Files:**
- vite.config.ts (47 lines) - Legacy configuration file
- vite.config.mts (70 lines) - Unused "main" configuration file

**Updated Documentation:**
- BUILDING.md - Corrected Vite configuration section to show actual usage

**Current Configuration Structure:**
- vite.config.web.mts - Used by build-web.sh
- vite.config.electron.mts - Used by build-electron.sh
- vite.config.capacitor.mts - Used by npm run build:capacitor
- vite.config.common.mts - Shared configuration utilities
- vite.config.utils.mts - Configuration utility functions

**Benefits:**
- Eliminates confusion about which config files to use
- Removes 117 lines of unused configuration code
- Documentation now matches actual build system usage
- Cleaner, more maintainable configuration structure

**Impact:**
- No functional changes to build process
- All platform builds continue to work correctly
- Reduced configuration complexity and maintenance overhead

BUILDING.md

@@ -2030,8 +2030,11 @@ share_target: {
 
 ### B.6 Additional Vite Configurations
 
-**vite.config.ts**: Legacy configuration file (minimal)
-**vite.config.mts**: Main configuration entry point
+**vite.config.web.mts**: Web platform configuration (used by build-web.sh)
+**vite.config.electron.mts**: Electron platform configuration (used by build-electron.sh)
+**vite.config.capacitor.mts**: Capacitor mobile configuration (used by npm run build:capacitor)
+**vite.config.common.mts**: Shared configuration utilities
+**vite.config.utils.mts**: Configuration utility functions
 **vite.config.dev.mts**: Development-specific configuration
 **vite.config.optimized.mts**: Optimized build configuration