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/en.ts

@@ -74,6 +74,7 @@ export const en: Translations = {
       documentation: "Documentation",
       keys: "Keys",
       logs: "Logs",
+      models: "Models",
       sessions: "Sessions",
       skills: "Skills",
     },
@@ -172,6 +173,18 @@ export const en: Translations = {
     inOut: "{input} in / {output} out",
   },
 
+  models: {
+    modelsUsed: "Models Used",
+    estimatedCost: "Est. Cost",
+    tokens: "tokens",
+    sessions: "sessions",
+    avgPerSession: "avg/session",
+    apiCalls: "API calls",
+    toolCalls: "tool calls",
+    noModelsData: "No model usage data for this period",
+    startSession: "Start a session to see model data here",
+  },
+
   logs: {
     title: "Logs",
     autoRefresh: "Auto-refresh",