feat(analytics): add usage analytics page with caching layer

- Add Analytics page with usage trends, model breakdown, sessions table
- Add server-side caching layer for better-ccusage data (TTL-based)
- Add request coalescing to prevent duplicate concurrent API calls
- Add /api/usage/refresh endpoint to manually clear cache
- Add date-range filter, summary cards, trend charts components
- Fix API parameter mismatch (since/until in YYYYMMDD format)
- Wire up Refresh button with loading state animation

src/types/external.d.ts

@@ -2,6 +2,81 @@
  * Type shims for incomplete external dependencies
  */
 
+// better-ccusage types (package has JS exports but incomplete TS subpath support)
+declare module 'better-ccusage/data-loader' {
+  export interface ModelBreakdown {
+    modelName: string;
+    inputTokens: number;
+    outputTokens: number;
+    cacheCreationTokens: number;
+    cacheReadTokens: number;
+    cost: number;
+  }
+
+  export interface DailyUsage {
+    date: string;
+    source: string;
+    inputTokens: number;
+    outputTokens: number;
+    cacheCreationTokens: number;
+    cacheReadTokens: number;
+    cost: number;
+    totalCost: number;
+    modelsUsed: string[];
+    modelBreakdowns: ModelBreakdown[];
+  }
+
+  export interface MonthlyUsage {
+    month: string;
+    source: string;
+    inputTokens: number;
+    outputTokens: number;
+    cacheCreationTokens: number;
+    cacheReadTokens: number;
+    totalCost: number;
+    modelsUsed: string[];
+    modelBreakdowns: ModelBreakdown[];
+  }
+
+  export interface SessionUsage {
+    sessionId: string;
+    projectPath: string;
+    inputTokens: number;
+    outputTokens: number;
+    cacheCreationTokens: number;
+    cacheReadTokens: number;
+    cost: number;
+    totalCost: number;
+    lastActivity: string;
+    versions: string[];
+    modelsUsed: string[];
+    modelBreakdowns: ModelBreakdown[];
+    source: string;
+  }
+
+  export interface DataLoaderOptions {
+    mode?: 'calculate' | 'cached';
+    claudePaths?: string[];
+  }
+
+  export function loadDailyUsageData(options?: DataLoaderOptions): Promise<DailyUsage[]>;
+  export function loadMonthlyUsageData(options?: DataLoaderOptions): Promise<MonthlyUsage[]>;
+  export function loadSessionData(options?: DataLoaderOptions): Promise<SessionUsage[]>;
+}
+
+declare module 'better-ccusage/calculate-cost' {
+  export interface Totals {
+    inputTokens: number;
+    outputTokens: number;
+    cacheCreationTokens: number;
+    cacheReadTokens: number;
+    costUSD: number;
+  }
+
+  export function calculateTotals(entries: unknown[]): Totals;
+  export function getTotalTokens(entries: unknown[]): number;
+}
+
 declare module 'cli-table3' {
   interface TableOptions {
     head?: string[];