feat: increase batch size limits to 15 and add testing_batch_size setting

Batch size configuration:
- Increase coding agent batch size limit from 1-3 to 1-15
- Increase testing agent batch size limit from 1-5 to 1-15
- Add separate `testing_batch_size` setting (previously only CLI-configurable)
- Pass testing_batch_size through full stack: schema → settings router →
  agent router → process manager → CLI flag

UI changes:
- Replace 3-button batch size selector with range slider (1-15)
- Add new Slider component (ui/src/components/ui/slider.tsx)
- Add "Features per Testing Agent" slider in settings panel
- Add custom slider CSS styling for webkit and mozilla

Updated across: CLAUDE.md, autonomous_agent_demo.py, parallel_orchestrator.py,
server/{schemas,routers,services}, and UI types/hooks/components.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/hooks/useProjects.ts

@@ -302,6 +302,7 @@ const DEFAULT_SETTINGS: Settings = {
   testing_agent_ratio: 1,
   playwright_headless: true,
   batch_size: 3,
+  testing_batch_size: 3,
   api_provider: 'claude',
   api_base_url: null,
   api_has_auth_token: false,