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/hooks/useProjects.ts

@@ -48,6 +48,20 @@ export function useDeleteProject() {
   })
 }
 
+export function useResetProject(projectName: string) {
+  const queryClient = useQueryClient()
+
+  return useMutation({
+    mutationFn: (fullReset: boolean) => api.resetProject(projectName, fullReset),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['projects'] })
+      queryClient.invalidateQueries({ queryKey: ['project', projectName] })
+      queryClient.invalidateQueries({ queryKey: ['features', projectName] })
+      queryClient.invalidateQueries({ queryKey: ['agent-status', projectName] })
+    },
+  })
+}
+
 export function useUpdateProjectSettings(projectName: string) {
   const queryClient = useQueryClient()