feat: 看板优化——中文标题+Token消耗+时间段筛选+品牌名统一

- 学员端：LoginPanel/App.vue 描述文案优化，去掉生硬表述
- 后台品牌统一：index.html/App.vue 标题改为'大本营千问千答'
- 看板tab：Dashboard -> 数据看板
- 看板新增：输入Token、输出Token、总Token 统计
- 看板支持按时间段筛选：后端API增加start/end参数，前端加日期选择器
- stats-grid布局从5列调整为4列，响应式改为2列

ai_knowledge_base_v2/apps/backend/app/api/admin_dashboard.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
-from fastapi import APIRouter, Depends
+from datetime import datetime
+
+from fastapi import APIRouter, Depends, Query
 from sqlalchemy.orm import Session
 
 from app.core.database import get_db
@@ -15,8 +17,16 @@ router = APIRouter()
 
 @router.get("/dashboard")
 def dashboard(
+    start: str = Query(default="", description="开始日期 YYYY-MM-DD"),
+    end: str = Query(default="", description="结束日期 YYYY-MM-DD"),
     db: Session = Depends(get_db),
     current_admin: Admin = Depends(get_current_admin),
 ) -> dict:
-    stats = AdminDashboardService.stats(db)
+    start_dt = datetime.strptime(start, "%Y-%m-%d") if start else None
+    if end:
+        end_dt = datetime.strptime(end, "%Y-%m-%d")
+        end_dt = end_dt.replace(hour=23, minute=59, second=59)
+    else:
+        end_dt = None
+    stats = AdminDashboardService.stats(db, start_dt, end_dt)
     return api_success(DashboardStats.model_validate(stats).model_dump())