ui: port cursor-chat-browser-python style and feel

- Replace purple/indigo palette with Cursor's black/blue palette
  (--bg #0a0a0a, --bg-card #111111, --border #2a2a2a, accent #3b82f6)
- Switch from Google Inter font to system-ui font stack
- Replace hero + card grid projects page with page-header + table layout
  matching cursor-chat-browser-python exactly (Project/Sessions/Last Modified)
- Add "Export all" button in page header with last-export timestamp display
- Replace <header> with <nav class="navbar"> / .navbar-inner structure
- Add GitHub footer matching cursor-chat-browser-python
- Replace left-border message rows with card bubbles (.bubble-user / .bubble-ai)
  using Cursor's blue/dark color scheme
- Port sidebar items to button/card style with active=blue-fill,
  date group labels, and scroll fade masks
- Add highlight.js + marked.js for proper markdown and code rendering
- Add setWorkspaceMode() to toggle body class that hides footer and
  removes container padding in workspace view
- Keep all Claude-specific features: stat badges, toast notifications,
  confirm modal, loading bar, scroll-to-top, session metadata panel,
  search page, mobile hamburger sidebar

Made-with: Cursor

Author: wpak-ai