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/schemas.py

@@ -6,11 +6,20 @@ Request/Response models for the API endpoints.
 """
 
 import base64
+import sys
 from datetime import datetime
+from pathlib import Path
 from typing import Literal
 
 from pydantic import BaseModel, Field, field_validator
 
+# Import model constants from registry (single source of truth)
+_root = Path(__file__).parent.parent
+if str(_root) not in sys.path:
+    sys.path.insert(0, str(_root))
+
+from registry import AVAILABLE_MODELS, DEFAULT_MODEL, VALID_MODELS
+
 # ============================================================================
 # Project Schemas
 # ============================================================================
@@ -102,7 +111,16 @@ class FeatureListResponse(BaseModel):
 
 class AgentStartRequest(BaseModel):
     """Request schema for starting the agent."""
-    yolo_mode: bool = False
+    yolo_mode: bool | None = None  # None means use global settings
+    model: str | None = None  # None means use global settings
+
+    @field_validator('model')
+    @classmethod
+    def validate_model(cls, v: str | None) -> str | None:
+        """Validate model is in the allowed list."""
+        if v is not None and v not in VALID_MODELS:
+            raise ValueError(f"Invalid model. Must be one of: {VALID_MODELS}")
+        return v
 
 
 class AgentStatus(BaseModel):
@@ -111,6 +129,7 @@ class AgentStatus(BaseModel):
     pid: int | None = None
     started_at: datetime | None = None
     yolo_mode: bool = False
+    model: str | None = None  # Model being used by running agent
 
 
 class AgentActionResponse(BaseModel):
@@ -239,3 +258,41 @@ class CreateDirectoryRequest(BaseModel):
     """Request to create a new directory."""
     parent_path: str
     name: str = Field(..., min_length=1, max_length=255)
+
+
+# ============================================================================
+# Settings Schemas
+# ============================================================================
+
+# Note: VALID_MODELS and DEFAULT_MODEL are imported from registry at the top of this file
+
+
+class ModelInfo(BaseModel):
+    """Information about an available model."""
+    id: str
+    name: str
+
+
+class SettingsResponse(BaseModel):
+    """Response schema for global settings."""
+    yolo_mode: bool = False
+    model: str = DEFAULT_MODEL
+
+
+class ModelsResponse(BaseModel):
+    """Response schema for available models list."""
+    models: list[ModelInfo]
+    default: str
+
+
+class SettingsUpdate(BaseModel):
+    """Request schema for updating global settings."""
+    yolo_mode: bool | None = None
+    model: str | None = None
+
+    @field_validator('model')
+    @classmethod
+    def validate_model(cls, v: str | None) -> str | None:
+        if v is not None and v not in VALID_MODELS:
+            raise ValueError(f"Invalid model. Must be one of: {VALID_MODELS}")
+        return v