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/store/pinnedFlowsStore.ts

@@ -0,0 +1,113 @@
+import { create } from 'zustand'
+import { pinnedFlowsApi } from '@/api/pinnedFlows'
+import type { PinnedFlow } from '@/api/pinnedFlows'
+import { toast } from '@/lib/toast'
+
+interface PinnedFlowsState {
+  items: PinnedFlow[]
+  isLoaded: boolean
+  isLoading: boolean
+  isMutatingByTreeId: Record<string, boolean>
+  error: string | null
+
+  load: (force?: boolean) => Promise<void>
+  pin: (treeId: string) => Promise<void>
+  unpin: (treeId: string) => Promise<void>
+  toggle: (treeId: string) => void
+  isPinned: (treeId: string) => boolean
+}
+
+export const usePinnedFlowsStore = create<PinnedFlowsState>()((set, get) => ({
+  items: [],
+  isLoaded: false,
+  isLoading: false,
+  isMutatingByTreeId: {},
+  error: null,
+
+  load: async (force = false) => {
+    const state = get()
+    if (state.isLoaded && !force) return
+    if (state.isLoading) return
+
+    set({ isLoading: true, error: null })
+    try {
+      const data = await pinnedFlowsApi.list()
+      set({ items: data.items, isLoaded: true, isLoading: false })
+    } catch {
+      set({ error: 'Failed to load pinned flows', isLoading: false })
+    }
+  },
+
+  pin: async (treeId: string) => {
+    const state = get()
+    if (state.isMutatingByTreeId[treeId]) return
+
+    set({ isMutatingByTreeId: { ...state.isMutatingByTreeId, [treeId]: true } })
+
+    try {
+      await pinnedFlowsApi.pin(treeId)
+      const data = await pinnedFlowsApi.list()
+      set((s) => ({
+        items: data.items,
+        isMutatingByTreeId: { ...s.isMutatingByTreeId, [treeId]: false },
+      }))
+    } catch (err: unknown) {
+      const status = (err as { response?: { status?: number } })?.response?.status
+      if (status === 409) {
+        toast.error('Maximum of 15 favorites reached. Unpin a flow to add a new one.')
+      } else {
+        toast.error('Failed to pin flow')
+      }
+      set((s) => ({
+        isMutatingByTreeId: { ...s.isMutatingByTreeId, [treeId]: false },
+      }))
+    }
+  },
+
+  unpin: async (treeId: string) => {
+    const state = get()
+    if (state.isMutatingByTreeId[treeId]) return
+
+    const prevItems = state.items
+    set({
+      items: state.items.filter((f) => f.tree_id !== treeId),
+      isMutatingByTreeId: { ...state.isMutatingByTreeId, [treeId]: true },
+    })
+
+    try {
+      await pinnedFlowsApi.unpin(treeId)
+      set((s) => ({
+        isMutatingByTreeId: { ...s.isMutatingByTreeId, [treeId]: false },
+      }))
+    } catch {
+      toast.error('Failed to unpin flow')
+      set((s) => ({
+        items: prevItems,
+        isMutatingByTreeId: { ...s.isMutatingByTreeId, [treeId]: false },
+      }))
+    }
+  },
+
+  toggle: (treeId: string) => {
+    const state = get()
+    if (state.isPinned(treeId)) {
+      state.unpin(treeId)
+    } else {
+      state.pin(treeId)
+    }
+  },
+
+  isPinned: (treeId: string) => {
+    return get().items.some((f) => f.tree_id === treeId)
+  },
+}))
+
+export const selectPinnedTreeIds = (state: PinnedFlowsState): Set<string> =>
+  new Set(state.items.map((f) => f.tree_id))
+
+export const selectPinLoadingTreeIds = (state: PinnedFlowsState): Set<string> =>
+  new Set(
+    Object.entries(state.isMutatingByTreeId)
+      .filter(([, v]) => v)
+      .map(([k]) => k)
+  )