feat: Add global settings modal and simplify agent controls

Adds a settings system for global configuration with YOLO mode toggle and
model selection. Simplifies the agent control UI by removing redundant
status indicator and pause functionality.

## Settings System
- New SettingsModal with YOLO mode toggle and model selection
- Settings persisted in SQLite (registry.db) - shared across all projects
- Models fetched from API endpoint (/api/settings/models)
- Single source of truth for models in registry.py - easy to add new models
- Optimistic UI updates with rollback on error

## Agent Control Simplification
- Removed StatusIndicator ("STOPPED"/"RUNNING" label) - redundant
- Removed Pause/Resume buttons - just Start/Stop toggle now
- Start button shows flame icon with fiery gradient when YOLO mode enabled

## Code Review Fixes
- Added focus trap to SettingsModal for accessibility
- Fixed YOLO button color contrast (WCAG AA compliance)
- Added model validation to AgentStartRequest schema
- Added model to AgentStatus response
- Added aria-labels to all icon-only buttons
- Added role="radiogroup" to model selection
- Added loading indicator during settings save
- Added SQLite timeout (30s) and retry logic with exponential backoff
- Added thread-safe database engine initialization
- Added orphaned lock file cleanup on server startup

## Files Changed
- registry.py: Model config, Settings CRUD, SQLite improvements
- server/routers/settings.py: New settings API
- server/schemas.py: Settings schemas with validation
- server/services/process_manager.py: Model param, orphan cleanup
- ui/src/components/SettingsModal.tsx: New modal component
- ui/src/components/AgentControl.tsx: Simplified to Start/Stop only

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/routers/agent.py

@@ -26,6 +26,21 @@ def _get_project_path(project_name: str) -> Path:
     return get_project_path(project_name)
 
 
+def _get_settings_defaults() -> tuple[bool, str]:
+    """Get YOLO mode and model defaults from global settings."""
+    import sys
+    root = Path(__file__).parent.parent.parent
+    if str(root) not in sys.path:
+        sys.path.insert(0, str(root))
+
+    from registry import DEFAULT_MODEL, get_all_settings
+
+    settings = get_all_settings()
+    yolo_mode = (settings.get("yolo_mode") or "false").lower() == "true"
+    model = settings.get("model", DEFAULT_MODEL)
+    return yolo_mode, model
+
+
 router = APIRouter(prefix="/api/projects/{project_name}/agent", tags=["agent"])
 
 # Root directory for process manager
@@ -69,6 +84,7 @@ async def get_agent_status(project_name: str):
         pid=manager.pid,
         started_at=manager.started_at,
         yolo_mode=manager.yolo_mode,
+        model=manager.model,
     )
 
 
@@ -80,7 +96,12 @@ async def start_agent(
     """Start the agent for a project."""
     manager = get_project_manager(project_name)
 
-    success, message = await manager.start(yolo_mode=request.yolo_mode)
+    # Get defaults from global settings if not provided in request
+    default_yolo, default_model = _get_settings_defaults()
+    yolo_mode = request.yolo_mode if request.yolo_mode is not None else default_yolo
+    model = request.model if request.model else default_model
+
+    success, message = await manager.start(yolo_mode=yolo_mode, model=model)
 
     return AgentActionResponse(
         success=success,