Add pure filter/lock logic with Vitest tests

Adds src/app/table-filters.ts (DataRow type, FilterType, distinctValues,
inferFilterType, matchesFilter, nextLockedRow) with 16 Vitest tests covering
type inference thresholds, date-range encoding, and row-lock toggling.

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

package.json

@@ -6,7 +6,9 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "@ant-design/icons": "^6.2.5",
@@ -29,6 +31,7 @@
     "eslint": "^9",
     "eslint-config-next": "16.2.9",
     "tailwindcss": "^4",
-    "typescript": "^5"
+    "typescript": "^5",
+    "vitest": "^4.1.9"
   }
 }