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>

vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    environment: "node",
+    include: ["src/**/*.test.ts"],
+  },
+});