Add search and LaTeX support

[DeepCogNeural]

Summary

• Add Cmd/Ctrl+F search with match count, next/previous navigation, Escape close, and highlighting in the Milkdown editor.
• Add LaTeX support using Milkdown math + KaTeX, including an Insert Formula modal via Cmd/Ctrl+Shift+E and click-to-edit rendered formulas.
• Preserve v1.5.0 slides/source mode: search targets the visible source textarea, and formula insertion writes Markdown into the textarea there.
• Include KaTeX styling in the editor bundle and HTML export output.

Testing

• npx tsc -p tsconfig.preload.json --noEmit
• npx tsc -p tsconfig.renderer.json --noEmit
• npm run build
• Playwright smoke test against the built renderer with mocked Electron APIs:
  • editor search opens, finds Welcome, and closes with Escape
  • source mode search supports typed queries without losing focus
  • formula modal inserts, renders, and serializes $E=mc^2$
  • source mode formula insertion saves Markdown text
  • HTML export includes KaTeX CSS

Note

npx tsc -p tsconfig.main.json --noEmit still fails on current main due to existing main-process type issues (fs.createServer and PrintToPDFOptions.marginType). This PR does not change those paths.