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

bun.lock

@@ -4,6 +4,7 @@
     "": {
       "name": "@kaitranntt/ccs",
       "dependencies": {
+        "better-ccusage": "^1.2.6",
         "boxen": "^8.0.1",
         "chalk": "^5.6.2",
         "chokidar": "^5.0.0",
@@ -445,6 +446,8 @@
 
     "before-after-hook": ["before-after-hook@4.0.0", "", {}, "sha512-q6tR3RPqIB1pMiTRMFcZwuG5T8vwp+vUvEG0vuI6B+Rikh5BfPp2fQ82c925FOs+b0lcFQ8CFrL+KbilfZFhOQ=="],
 
+    "better-ccusage": ["better-ccusage@1.2.6", "", { "bin": { "better-ccusage": "dist/index.js" } }, "sha512-IZCYBX1kF0IfJ6ho9JMwLKn2o820WRiVGZ+2tVS2olODU5J7Np5mJ1j1i5HtazZPNo2S9wKU9C9iysc0f8Cjqw=="],
+
     "body-parser": ["body-parser@1.20.4", "", { "dependencies": { "bytes": "~3.1.2", "content-type": "~1.0.5", "debug": "2.6.9", "depd": "2.0.0", "destroy": "~1.2.0", "http-errors": "~2.0.1", "iconv-lite": "~0.4.24", "on-finished": "~2.4.1", "qs": "~6.14.0", "raw-body": "~2.5.3", "type-is": "~1.6.18", "unpipe": "~1.0.0" } }, "sha512-ZTgYYLMOXY9qKU/57FAo8F+HA2dGX7bqGc71txDRC1rS4frdFI5R7NhluHxH6M0YItAP0sHB4uqAOcYKxO6uGA=="],
 
     "bottleneck": ["bottleneck@2.19.5", "", {}, "sha512-VHiNCbI1lKdl44tGrhNfU3lup0Tj/ZBMJB5/2ZbNXRCPuRCO7ed2mgcK4r17y+KB2EfuYuRaVlwNbAeaWGSpbw=="],