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/package.json

@@ -25,6 +25,7 @@
   },
   "devDependencies": {
     "@repo/core-eslint": "workspace:*",
+    "@repo/core-testing": "workspace:*",
     "@repo/core-typescript": "workspace:*",
     "@types/node": "^22.0.0",
     "vitest": "^3.1.0"