feat(audit): Batch 24 — one validate-and-coerce schema layer for settings (CC9)

zod field schemas in settingsSchema.ts replace setSettings' bespoke
per-key switch: parse → skip-on-fail → store, side effects (secret
encryption, launch-at-startup sync) kept in settings.ts. sanitizeOptions
extracted to settingsOptions.ts to break the import cycle; backup import
flows through the same schema. Field-by-field parity pinned in
test/settingsSchema.test.ts (15 tests); full suite unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -40,6 +40,7 @@
     "electron-store": "^11.0.2",
     "react": "^19.2.7",
     "react-dom": "^19.2.7",
+    "zod": "^4.4.3",
     "zustand": "^5.0.14"
   },
   "devDependencies": {