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/settings.py

@@ -0,0 +1,75 @@
+"""
+Settings Router
+===============
+
+API endpoints for global settings management.
+Settings are stored in the registry database and shared across all projects.
+"""
+
+import sys
+from pathlib import Path
+
+from fastapi import APIRouter
+
+from ..schemas import ModelInfo, ModelsResponse, SettingsResponse, SettingsUpdate
+
+# Add root to path for registry import
+ROOT_DIR = Path(__file__).parent.parent.parent
+if str(ROOT_DIR) not in sys.path:
+    sys.path.insert(0, str(ROOT_DIR))
+
+from registry import (
+    AVAILABLE_MODELS,
+    DEFAULT_MODEL,
+    DEFAULT_YOLO_MODE,
+    get_all_settings,
+    set_setting,
+)
+
+router = APIRouter(prefix="/api/settings", tags=["settings"])
+
+
+def _parse_yolo_mode(value: str | None) -> bool:
+    """Parse YOLO mode string to boolean."""
+    return (value or "false").lower() == "true"
+
+
+@router.get("/models", response_model=ModelsResponse)
+async def get_available_models():
+    """Get list of available models.
+
+    Frontend should call this to get the current list of models
+    instead of hardcoding them.
+    """
+    return ModelsResponse(
+        models=[ModelInfo(id=m["id"], name=m["name"]) for m in AVAILABLE_MODELS],
+        default=DEFAULT_MODEL,
+    )
+
+
+@router.get("", response_model=SettingsResponse)
+async def get_settings():
+    """Get current global settings."""
+    all_settings = get_all_settings()
+
+    return SettingsResponse(
+        yolo_mode=_parse_yolo_mode(all_settings.get("yolo_mode")),
+        model=all_settings.get("model", DEFAULT_MODEL),
+    )
+
+
+@router.patch("", response_model=SettingsResponse)
+async def update_settings(update: SettingsUpdate):
+    """Update global settings."""
+    if update.yolo_mode is not None:
+        set_setting("yolo_mode", "true" if update.yolo_mode else "false")
+
+    if update.model is not None:
+        set_setting("model", update.model)
+
+    # Return updated settings
+    all_settings = get_all_settings()
+    return SettingsResponse(
+        yolo_mode=_parse_yolo_mode(all_settings.get("yolo_mode")),
+        model=all_settings.get("model", DEFAULT_MODEL),
+    )