feat: Phase 3 — Webpack → electron-vite migration

- Add electron.vite.config.ts (main/preload/renderer three-target build)
- Custom Vite plugins: mdRawPlugin (.md as raw string), snapSvgPlugin (UMD→ESM),
  svgSpritePlugin (svg-sprite-loader replacement)
- Regex aliases for Node.js stubs (exact matching: /^fs$/ not 'fs')
- Node.js browser stubs in src/renderer/node/ (fs, fs-extra, fs/promises,
  child_process, zlib) — already committed with Phase 2
- All three targets share dist/electron/; main cleans (emptyOutDir:true),
  others set false to avoid overwriting each other
- src/renderer/index.html (Vite native HTML entry, replaces src/index.ejs)
- notification/index.js: html?raw suffix (replaces webpack html-loader)
- dompurify.js: destructure from default import (no named exports in purify.es.js)
- Remove .electron-vue/webpack.main.config.js, webpack.renderer.config.js,
  dev-client.js (webpack configs no longer needed)
- package.json: electron-vite scripts, remove webpack/babel devDeps, add
  vite plugins, update electron-builder compat

Build: electron-vite build passes all 3 targets (main 244 modules, renderer ~3313).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>