tests: add coverage for src/actions/ (15 files) and src/lib/{admin,auth} (3 files)

- src/actions coverage: 2.4% -> 13.55%
- src/lib/admin coverage: 44.3% -> 84.81%
- src/lib/auth coverage: 90.52%
- vitest.config.ts: exclude .next.prev/ from test discovery

vitest.config.ts

@@ -18,7 +18,7 @@ export default defineConfig({
 		// Modules under test transitively import @/env; skip its strict parse so
 		// unit tests run without a populated .env.
 		env: { SKIP_ENV_VALIDATION: "1" },
-		exclude: ["e2e/**", "node_modules/**", ".next/**"],
+		exclude: ["e2e/**", "node_modules/**", ".next/**", ".next.prev/**"],
 		coverage: {
 			enabled: true,
 			provider: "v8",