feat(v1.3.0): add ephemeral session-key encryption for sensitive state

- Add src/lib/sessionCrypto.ts with AES-GCM-256 session keys
- Integrate into Backup flow: auto-clear plaintext mnemonic after QR gen
- Add Lock/Clear button to destroy key and clear all state
- Add cleanup on component unmount
- Fix unused imports and TypeScript strict mode errors

package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "dev": "vite",
     "build": "tsc && vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "typecheck": "tsc --noEmit"
   },
   "dependencies": {
     "html5-qrcode": "^2.3.8",