feat: 首页 Go SSR 与 React hydrate 同构，消壳层与帖行闪动

补齐侧栏/右栏图标与徽章、鉴权种子、StaticFeedList，并修正嵌套 a 与标题徽章对齐。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/utils/authBoot.ts

@@ -0,0 +1,43 @@
+import type { User } from '../api/types';
+
+/** 首页 SSR boot 注入的鉴权种子（consumeHomeBoot 写入；AuthProvider 仅 peek，保留至下次 boot） */
+let seededUser: User | null | undefined;
+let seededUnread = 0;
+let hasAuthSeed = false;
+/** 未读数可被 MainLayout 同步读取 */
+let bootUnread = 0;
+
+export function seedAuthFromHomeBoot(user: User | null | undefined, unread: number) {
+  hasAuthSeed = true;
+  seededUser = user ?? null;
+  seededUnread = Math.max(0, unread | 0);
+  bootUnread = seededUnread;
+}
+
+type AuthSeed = { hasSeed: boolean; user: User | null; unread: number };
+
+/** 只读种子，不清空（StrictMode 双挂 / useState initializer 可重复调用） */
+export function peekAuthSeed(): AuthSeed {
+  if (!hasAuthSeed) {
+    return { hasSeed: false, user: null, unread: bootUnread };
+  }
+  return { hasSeed: true, user: seededUser ?? null, unread: seededUnread };
+}
+
+/** mount 后清除用户种子，避免后续误用 */
+export function clearAuthSeed() {
+  hasAuthSeed = false;
+  seededUser = undefined;
+  seededUnread = 0;
+}
+
+/** @deprecated 改用 peekAuthSeed + clearAuthSeed；保留兼容 */
+export function takeAuthSeed(): AuthSeed {
+  const s = peekAuthSeed();
+  if (s.hasSeed) clearAuthSeed();
+  return s;
+}
+
+export function getBootUnread(): number {
+  return bootUnread;
+}