fix: resolve runtime startup failures — global→window, CSS @import order

- Replace `global.marktext` → `window.marktext` across 11 renderer files
  (Vite's `define: { global: 'window' }` was ineffective with nodePolyfills)
- Move CSS `@import url("prismjs/light.theme.css")` to top of default.css
  (PostCSS requires @import before all other rules)
- Exclude `fontmanager-redux` from Vite dep scan (native module)
- Remove dead `global: 'window'` define from electron.vite.config.ts

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