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>

server/services/assistant_database.py

@@ -79,6 +79,26 @@ def get_engine(project_dir: Path):
     return _engine_cache[cache_key]
 
 
+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()
+        logger.debug(f"Disposed database engine for {cache_key}")
+        return True
+
+    return False
+
+
 def get_session(project_dir: Path):
     """Get a new database session for a project."""
     engine = get_engine(project_dir)