refactor: shard monolithic index.html into separate CSS/JS files

[Oaklight]

Summary

• Extract all CSS (~1318 lines) from inline <style> into static/css/app.css
• Extract all JS (~4028 lines) from inline <script> into static/js/app.js
• Slim index.html from ~5700 lines to ~356 lines (markup only)
• Add /static/ route in server with MIME type auto-detection and path traversal protection
• Migrate inline onclick handlers to addEventListener for ES module compatibility

Motivation

Every frontend feature touched the same 5700-line file, causing heavy merge conflicts and defeating browser caching for CSS/JS. This is a pure restructuring with no behavior change.

Test plan

• Page loads correctly with separate CSS/JS (200 OK, correct MIME types)
• Zero console errors (entire session)
• File tree, editor (CodeMirror), PDF preview all functional
• Git panel, search panel, outline panel working
• Layout switching (Editor/Split/PDF) + localStorage persistence
• Settings popup, theme switching, i18n
• Quick Open (Ctrl+P), sidebar toggle (Ctrl+B), keyboard shortcuts popup
• Compile log toggle + Copy button (onclick→addEventListener migration)
• Path traversal protection (../ attacks return 404)

Closes #8