feat(core-typescript): split vitest base into node + jsdom flavors

Adds vitest.base.node and vitest.base.jsdom with safety defaults
(clearMocks, restoreMocks, mockReset, unstubGlobals, sequence.shuffle)
and coverage thresholds (80/75/80/80). Migrates all feature configs
to the new base. Existing baseVitestConfig kept as backwards-compat
re-export of nodeVitestConfig.

Spec: §6.2

packages/auth/vitest.config.ts

@@ -1,11 +1,9 @@
 import path from "node:path";
-import { baseVitestConfig } from "@repo/core-typescript/vitest.base";
+import { mergeConfig } from "vitest/config";
+import { nodeVitestConfig } from "@repo/core-typescript/vitest.base.node";
 
-export default {
-  ...baseVitestConfig,
+export default mergeConfig(nodeVitestConfig, {
   resolve: {
-    alias: {
-      "@": path.resolve(__dirname, "./src"),
-    },
+    alias: { "@": path.resolve(__dirname, "./src") },
   },
-};
+});