feat: useWallet composable — NWC + Lightning Address wallet management

Client-side wallet composable with NWC auto-pay, invoice polling,
Cashu token submission, and persistent wallet state. Follows useNostr
pattern with readonly exports and localStorage persistence.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -9,6 +9,7 @@
   },
   "dependencies": {
     "kaplay": "^3001.0.19",
+    "nostr-tools": "^2.23.3",
     "vue": "^3.5.13",
     "vue-router": "^4.5.1"
   },