feat: UI redesign v0.0.2 — color unification, navigation improvements, mobile support

- Unify color palette from mixed green/blue/purple to consistent green theme
- Sidebar: add text labels to create/join group buttons for discoverability
- Header: add quick action buttons (create group, join group, notifications)
- Mobile: add hamburger menu with slide-out sidebar and overlay
- Home: add prominent CTA buttons, onboarding card for empty state
- Join group dialog: add search-by-name mode alongside existing ID lookup
- Games library: inline group selector dropdown instead of external selection

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/api/groups.ts

@@ -38,6 +38,20 @@ export async function getGroup(groupId: string): Promise<Group> {
   }) as unknown as Group
 }
 
+// 按名称搜索群组
+export async function searchGroups(keyword: string): Promise<Group[]> {
+  if (!keyword.trim()) return []
+
+  const user = pb.authStore.model
+  const filter = `name ~ "${keyword.trim()}" && id != "${user?.id}"`
+
+  const result = await pb.collection('groups').getList(1, 20, {
+    filter,
+    $autoCancel: false
+  })
+  return result.items as unknown as Group[]
+}
+
 // 直接加入群组（无需审核时调用）
 export async function joinGroup(groupId: string) {
   const user = pb.authStore.model