feat: modernize Electron build process with Vite-based CSS injection

- Replace manual CSS injection hack with Vite plugin
- Configure Vite to handle both main process and renderer builds
- Update build scripts to work with proper Vite output structure
- Remove fix-inject-css.js post-build script
- Update BUILDING.md documentation
- Add build-modernization-context.md for future reference

Technical changes:
- vite.config.electron.mts: Add electron-css-injection plugin and proper output config
- scripts/build-electron.js: Simplify to work with Vite-generated files
- BUILDING.md: Update Electron build documentation
- doc/build-modernization-context.md: Document context and decisions

Security/maintenance improvements:
- Eliminate manual file manipulation hacks
- Ensure deterministic, reproducible builds
- Centralize build logic in Vite configuration
- Improve developer experience and CI/CD compatibility

Author: Matthew Raymer

vite.config.electron.renderer.mts

@@ -14,12 +14,14 @@ export default defineConfig({
     outDir: 'dist-electron/www',
     emptyOutDir: false,
     assetsDir: 'assets',
+    cssCodeSplit: false,
     rollupOptions: {
       input: path.resolve(__dirname, 'src/main.electron.ts'),
       output: {
         entryFileNames: 'main.electron.js',
         assetFileNames: 'assets/[name]-[hash][extname]',
-        chunkFileNames: 'assets/[name]-[hash].js'
+        chunkFileNames: 'assets/[name]-[hash].js',
+        manualChunks: undefined
       }
     },
     commonjsOptions: {