feat: add project reset functionality with quick and full reset options

Add the ability to reset a project to its initial state with two options:
- Quick Reset: Clears features.db, assistant.db, and settings files while
  preserving app spec and prompts
- Full Reset: Deletes everything including prompts directory, triggering
  the setup wizard for project reconfiguration

Backend changes:
- Add POST /{name}/reset endpoint to projects router with full_reset query param
- Validate agent lock file to prevent reset while agent is running (409 Conflict)
- Dispose database engines before deleting files to release Windows file locks
- Add engine caching to api/database.py for better connection management
- Add dispose_engine() functions to both database modules
- Delete WAL mode journal files (*.db-wal, *.db-shm) during reset

Frontend changes:
- Add ResetProjectModal component with toggle between Quick/Full reset modes
- Add ProjectSetupRequired component shown when has_spec is false
- Add resetProject API function and useResetProject React Query hook
- Integrate reset button in header (disabled when agent running)
- Add 'R' keyboard shortcut to open reset modal
- Show ProjectSetupRequired when project needs setup after full reset

This implements the feature from PR #4 directly on master to avoid merge
conflicts.

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

api/database.py

@@ -336,12 +336,20 @@ def create_database(project_dir: Path) -> tuple:
     """
     Create database and return engine + session maker.
 
+    Uses a cache to avoid creating new engines for each request, which improves
+    performance by reusing database connections.
+
     Args:
         project_dir: Directory containing the project
 
     Returns:
         Tuple of (engine, SessionLocal)
     """
+    cache_key = project_dir.as_posix()
+
+    if cache_key in _engine_cache:
+        return _engine_cache[cache_key]
+
     db_url = get_database_url(project_dir)
     engine = create_engine(db_url, connect_args={
         "check_same_thread": False,
@@ -369,12 +377,39 @@ def create_database(project_dir: Path) -> tuple:
     _migrate_add_schedules_tables(engine)
 
     SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+    # Cache the engine and session maker
+    _engine_cache[cache_key] = (engine, SessionLocal)
+
     return engine, SessionLocal
 
 
+def dispose_engine(project_dir: Path) -> bool:
+    """Dispose of and remove the cached engine for a project.
+
+    This closes all database connections, releasing file locks on Windows.
+    Should be called before deleting the database file.
+
+    Returns:
+        True if an engine was disposed, False if no engine was cached.
+    """
+    cache_key = project_dir.as_posix()
+
+    if cache_key in _engine_cache:
+        engine, _ = _engine_cache.pop(cache_key)
+        engine.dispose()
+        return True
+
+    return False
+
+
 # Global session maker - will be set when server starts
 _session_maker: Optional[sessionmaker] = None
 
+# Engine cache to avoid creating new engines for each request
+# Key: project directory path (as posix string), Value: (engine, SessionLocal)
+_engine_cache: dict[str, tuple] = {}
+
 
 def set_session_maker(session_maker: sessionmaker) -> None:
     """Set the global session maker."""