feat: add Models dashboard tab with rich per-model analytics

- New /models page in left nav (after Analytics)
- New /api/analytics/models endpoint with per-model token/cost/session
  breakdown, cache read/reasoning tokens, tool calls, avg tokens/session,
  and capabilities from models.dev (vision/tools/reasoning/context window)
- Model cards with stacked token distribution bar, capability badges,
  provider badges, cost info, and relative time
- Summary stats bar (models used, total tokens, est. cost, sessions)
- Period selector (7d/30d/90d) with refresh
- i18n support (en + zh)

web/src/i18n/zh.ts

@@ -73,6 +73,7 @@ export const zh: Translations = {
       documentation: "文档",
       keys: "密钥",
       logs: "日志",
+      models: "模型",
       sessions: "会话",
       skills: "技能",
     },
@@ -170,6 +171,18 @@ export const zh: Translations = {
     inOut: "输入 {input} / 输出 {output}",
   },
 
+  models: {
+    modelsUsed: "使用模型数",
+    estimatedCost: "预估费用",
+    tokens: "Token",
+    sessions: "会话",
+    avgPerSession: "平均/会话",
+    apiCalls: "API 调用",
+    toolCalls: "工具调用",
+    noModelsData: "该时间段暂无模型使用数据",
+    startSession: "开始会话后将在此显示模型数据",
+  },
+
   logs: {
     title: "日志",
     autoRefresh: "自动刷新",