fix: abort dangling fetches and handle localStorage exceptions

- useNostr.ts: wrap auto-restore login fetch with AbortController,
  abort on logout to cancel in-flight request
- useWallet.ts: wrap localStorage.setItem/removeItem calls in try/catch
  for Safari private browsing quota exceptions

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

frontend/src/composables/useWallet.ts

@@ -16,8 +16,10 @@ function loadStored<T>(key: string): T | null {
   } catch { return null }
 }
 function store(key: string, value: unknown) {
-  if (value == null) localStorage.removeItem(key)
-  else localStorage.setItem(key, JSON.stringify(value))
+  try {
+    if (value == null) localStorage.removeItem(key)
+    else localStorage.setItem(key, JSON.stringify(value))
+  } catch { /* Safari private browsing quota exception */ }
 }
 
 const isWalletConnected = ref(!!loadStored<string>('bf_wallet_method'))
@@ -73,7 +75,7 @@ export function useWallet() {
     }
 
     // Store NWC string locally for client-side payment sending
-    localStorage.setItem('bf_nwc_url', connectionString)
+    try { localStorage.setItem('bf_nwc_url', connectionString) } catch { /* quota */ }
     walletMethod.value = 'nwc'
     isWalletConnected.value = true
     store('bf_wallet_method', 'nwc')
@@ -120,7 +122,7 @@ export function useWallet() {
     paymentStatus.value = 'idle'
     pendingPayment.value = null
     store('bf_wallet_method', null)
-    localStorage.removeItem('bf_nwc_url')
+    try { localStorage.removeItem('bf_nwc_url') } catch { /* quota */ }
   }
 
   async function checkWalletStatus(): Promise<void> {