feat: add token usage statistics with daily tracking and stats modal

- Add TokenUsageEntry and TokenStats types for daily tracking
- Add state-store methods for recording and retrieving daily usage
- Add /api/token-stats endpoint with daily breakdown and totals
- Add token stats modal with summary cards, 7-day bar chart, and table
- Record tokens periodically (5min) and on session delete
- Make header tokens clickable to open stats modal
- Fix subagents panel toggle icon direction (down when open)
- Increase subagents panel height when expanded

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/types.ts

@@ -109,6 +109,34 @@ export interface GlobalStats {
   lastUpdatedAt: number;
 }
 
+// ========== Token Usage History Types ==========
+
+/**
+ * Daily token usage entry for historical tracking.
+ */
+export interface TokenUsageEntry {
+  /** Date in YYYY-MM-DD format */
+  date: string;
+  /** Input tokens used on this day */
+  inputTokens: number;
+  /** Output tokens used on this day */
+  outputTokens: number;
+  /** Estimated cost in USD */
+  estimatedCost: number;
+  /** Number of sessions that contributed to this day's usage */
+  sessions: number;
+}
+
+/**
+ * Token usage statistics with daily tracking.
+ */
+export interface TokenStats {
+  /** Daily usage entries (most recent first) */
+  daily: TokenUsageEntry[];
+  /** Timestamp of last update */
+  lastUpdated: number;
+}
+
 // ========== Task Types ==========
 
 /**