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>

ui/src/lib/api.ts

@@ -111,6 +111,23 @@ export async function updateProjectSettings(
   })
 }
 
+export interface ResetProjectResponse {
+  success: boolean
+  reset_type: 'quick' | 'full'
+  deleted_files: string[]
+  message: string
+}
+
+export async function resetProject(
+  name: string,
+  fullReset: boolean = false
+): Promise<ResetProjectResponse> {
+  const params = fullReset ? '?full_reset=true' : ''
+  return fetchJSON(`/projects/${encodeURIComponent(name)}/reset${params}`, {
+    method: 'POST',
+  })
+}
+
 // ============================================================================
 // Features API
 // ============================================================================