feat(desktop): friendNicknames local store + useNickname hook

apps/desktop/src/lib/friendNicknames.ts

@@ -0,0 +1,64 @@
+import { useSyncExternalStore } from 'react';
+
+// Local-only friend nickname overrides. Stored in localStorage keyed by the
+// peer's user-id. Empty/missing value = use the real display name.
+// Local-only by design — friends never see your nickname for them.
+
+const STORAGE_KEY = 'chatapp.friendNicknames.v1';
+
+let cache: Record<string, string> | null = null;
+const listeners = new Set<() => void>();
+
+function load(): Record<string, string> {
+  if (cache) return cache;
+  try {
+    const raw = window.localStorage.getItem(STORAGE_KEY);
+    if (!raw) { cache = {}; return cache; }
+    const parsed = JSON.parse(raw) as unknown;
+    if (parsed && typeof parsed === 'object') {
+      cache = {};
+      for (const [k, v] of Object.entries(parsed as Record<string, unknown>)) {
+        if (typeof v === 'string' && v.trim().length > 0) cache[k] = v;
+      }
+      return cache;
+    }
+  } catch { /* corrupted; fall through */ }
+  cache = {};
+  return cache;
+}
+
+function persist(): void {
+  try { window.localStorage.setItem(STORAGE_KEY, JSON.stringify(cache ?? {})); }
+  catch { /* quota / private mode */ }
+  for (const l of listeners) l();
+}
+
+export function getNickname(userId: string): string | null {
+  return load()[userId] ?? null;
+}
+
+export function setNickname(userId: string, nickname: string | null): void {
+  const store = load();
+  const trimmed = nickname?.trim() ?? '';
+  if (trimmed.length === 0) {
+    if (!(userId in store)) return;
+    delete store[userId];
+  } else {
+    if (store[userId] === trimmed) return;
+    store[userId] = trimmed;
+  }
+  persist();
+}
+
+// Reactive hook: returns the current nickname for a user, or `fallback`
+// when no nickname is set. Re-renders when ANY nickname changes (cheap,
+// the set is small).
+export function useNickname(userId: string | null | undefined, fallback: string): string {
+  const subscribe = (cb: () => void) => {
+    listeners.add(cb);
+    return () => { listeners.delete(cb); };
+  };
+  const getSnapshot = () => (userId ? getNickname(userId) : null);
+  const nickname = useSyncExternalStore(subscribe, getSnapshot, getSnapshot);
+  return nickname ?? fallback;
+}