feat: consolidate dashboard pages, fix namespace links, add tests

Merge Overview and Full Audit into a single dashboard page that always
shows the skipped check count. Fix namespace link 404s by using
Headlamp's Link component (which generates cluster-prefixed URLs)
instead of raw react-router-dom Link. Add vitest unit tests for all
polaris.ts utility functions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vitest.config.mts

@@ -0,0 +1,8 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'jsdom',
+  },
+});