test: add comprehensive unit tests with Vitest

65 tests covering canvas decoder, pixel buffer, user ID hashing,
canvas storage (with binary encoding regression test), Redis client
API shape, and worker endpoint validation.

package.json

@@ -7,16 +7,20 @@
     "dev:client": "vite dev",
     "build": "vite build",
     "deploy": "vite build && wrangler deploy",
-    "preview": "vite build && wrangler dev"
+    "preview": "vite build && wrangler dev",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "@upstash/redis": "^1.34.3",
     "hono": "^4.7.6"
   },
   "devDependencies": {
+    "@cloudflare/vitest-pool-workers": "^0.14.7",
     "@sveltejs/vite-plugin-svelte": "^5.0.3",
     "svelte": "^5.28.2",
     "vite": "^6.3.2",
+    "vitest": "^4.1.4",
     "wrangler": "^4.14.1"
   }
 }