Add standalone debug page at /debug ...

Replaces the old_webclient/debug.html with a proper self-contained
tool at debug/index.html. New features over the old page:
- Sidebar session list with profile, message count, pin indicator
- Auto-refresh toggle (3s interval)
- Refresh button
- Renders thinking blocks, is_plan and is_summary tags
- Shows tool call name on tool result messages
- Clickable image thumbnails (open full-size)
- All new fields from the current LLM context API

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