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

src/main.electron.ts

@@ -1,3 +1,4 @@
+import './assets/styles/tailwind.css';
 import { initializeApp } from "./main.common";
 import { logger } from "./utils/logger";