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>

server/services/process_manager.py

@@ -374,6 +374,7 @@ class AgentProcessManager:
         testing_agent_ratio: int = 1,
         playwright_headless: bool = True,
         batch_size: int = 3,
+        testing_batch_size: int = 3,
     ) -> tuple[bool, str]:
         """
         Start the agent as a subprocess.
@@ -440,6 +441,9 @@ class AgentProcessManager:
         # Add --batch-size flag for multi-feature batching
         cmd.extend(["--batch-size", str(batch_size)])
 
+        # Add --testing-batch-size flag for testing agent batching
+        cmd.extend(["--testing-batch-size", str(testing_batch_size)])
+
         # Apply headless setting to .playwright/cli.config.json so playwright-cli
         # picks it up (the only mechanism it supports for headless control)
         self._apply_playwright_headless(playwright_headless)