feat: add Vitest unit tests for utility functions

- Set up Vitest with 27 tests across 3 test suites
- relativeTime: edge cases, time buckets, future timestamps
- sessionDisplayName: labels, kinds, channels, UUID truncation
- messagesToMarkdown: roles, blocks, tool calls, system events
- Add test and test:watch npm scripts
- Add test step to CI workflow

package.json

@@ -30,6 +30,8 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "preview": "vite preview",
     "lint:fix": "eslint . --fix"
   },
@@ -63,7 +65,8 @@
     "globals": "^16.5.0",
     "typescript": "~5.9.3",
     "typescript-eslint": "^8.48.0",
-    "vite": "^7.3.1"
+    "vite": "^7.3.1",
+    "vitest": "^4.0.18"
   },
   "engines": {
     "node": ">=18"