fix: remove Set-based Zustand selectors causing infinite re-render loop

Zustand selectors returning new Set() on every call fail Object.is
equality check, triggering continuous re-renders. Replaced with
useMemo-derived Sets in consuming components.

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

frontend/src/store/pinnedFlowsStore.ts

@@ -102,12 +102,3 @@ export const usePinnedFlowsStore = create<PinnedFlowsState>()((set, get) => ({
   },
 }))
 
-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)
-  )