feat: Phase 1 — pinnedFlowsStore, pagination hook, cached quota hook, sidebar refactor

- Add pin() to pinnedFlowsApi
- Create pinnedFlowsStore (Zustand) — single source of truth for pin state
- Add dashboardMyFlowsView preference to userPreferencesStore
- Create usePaginationParams hook (URL-synced)
- Create useCachedQuota hook (5-min TTL)
- Sidebar uses pinnedFlowsStore instead of local state

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/api/pinnedFlows.ts

@@ -25,6 +25,10 @@ export const pinnedFlowsApi = {
   unpin: async (treeId: string): Promise<void> => {
     await apiClient.delete(`/trees/${treeId}/pin`)
   },
+
+  pin: async (treeId: string): Promise<void> => {
+    await apiClient.post(`/trees/${treeId}/pin`)
+  },
 }
 
 export default pinnedFlowsApi