Initial scaffold: Researcher Endorsement frontend

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/agents/zustand-sentry.md

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+---
+name: "Zustand Sentry"
+description: "Generates a persisted Zustand store for endorsements and likes (localStorage)."
+triggers:
+  - "generate zustand store"
+  - "persist endorsements to localStorage"
+  - "create social store"
+---
+
+# Zustand Sentry
+
+Purpose
+- Manage social state (endorsements, likes) in the frontend using `zustand` + `persist` so no backend is required.
+
+Example prompts
+- "generate a zustand store for endorsements and likes"
+- "create a persisted store to save endorsements in localStorage"
+
+Output
+- Creates `src/stores/useSocialStore.ts` with a typed Zustand store (persisted to localStorage) that exposes: `endorsements`, `likes`, `addEndorsement`, `toggleLike`, and `reset`.
+
+Code (generated file: src/stores/useSocialStore.ts)
+```ts
+import create from 'zustand'
+import { persist } from 'zustand/middleware'
+
+type Endorsement = {
+  id: string
+  paperId: string
+  userId: string
+  message?: string
+  createdAt: string
+}
+
+type SocialState = {
+  endorsements: Record<string, Endorsement[]>
+  likes: Record<string, string[]>
+  addEndorsement: (paperId: string, endorser: Omit<Endorsement,'id'|'createdAt'>) => void
+  toggleLike: (paperId: string, userId: string) => void
+  reset: () => void
+}
+
+export const useSocialStore = create<SocialState>(persist(
+  (set) => ({
+    endorsements: {},
+    likes: {},
+    addEndorsement: (paperId, endorser) => set((state) => {
+      const list = state.endorsements[paperId] ?? []
+      const newItem = {
+        id: (typeof crypto !== 'undefined' && (crypto as any).randomUUID) ? (crypto as any).randomUUID() : Date.now().toString(),
+        ...endorser,
+        createdAt: new Date().toISOString()
+      }
+      return { endorsements: { ...state.endorsements, [paperId]: [...list, newItem] } }
+    }),
+    toggleLike: (paperId, userId) => set((state) => {
+      const users = state.likes[paperId] ?? []
+      const has = users.includes(userId)
+      return { likes: { ...state.likes, [paperId]: has ? users.filter(u => u !== userId) : [...users, userId] } }
+    }),
+    reset: () => ({ endorsements: {}, likes: {} })
+  }),
+  { name: 'arxiv-social-storage' }
+))
+
+export default useSocialStore
+```
+
+Notes
+- Recommend adding `zustand` and `zustand/middleware` to dependencies and documenting wiring examples in components.