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/useNostr.ts

@@ -90,12 +90,14 @@ export function useNostr() {
   const hasExtension = computed(() => !!window.nostr)
 
   // Restore session on first load — re-verify with server (once only)
+  const autoRestoreController = new AbortController()
   if (!autoRestoreRan && pubkey.value && !bot.value) {
     autoRestoreRan = true
     fetch('/api/auth/login', {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({ pubkey: pubkey.value }),
+      signal: autoRestoreController.signal,
     }).then(r => r.json()).then(data => {
       if (data.exists) {
         bot.value = normalizeBotData(data.bot)
@@ -368,6 +370,7 @@ export function useNostr() {
   }
 
   function logout() {
+    autoRestoreController.abort()
     clearAllState()
     // Don't clear bf_nsec on logout — user may want to log back in
   }