Harden accessibility: ARIA roles, semantic buttons, alt text, form labels

- Replace clickable divs with semantic <button> elements (NoteCard, SearchView)
- Add role="dialog", aria-modal, aria-labelledby on all modals (Login, Quote, Zap, Lightbox)
- Add role="presentation" on overlay backdrops (NoteCard menu, emoji pickers)
- Add aria-label to all icon-only buttons (close, nav arrows, context menu)
- Add aria-expanded to context menu toggle
- Add meaningful alt text to all 35+ images (avatars, covers, thumbnails, media)
- Add aria-label to form inputs (search, onboarding login)

src/components/shared/SettingsView.tsx

@@ -27,7 +27,7 @@ function MutedRow({ pubkey, onUnmute }: { pubkey: string; onUnmute: () => void }
   return (
     <div className="flex items-center gap-3 px-3 py-2 border border-border text-[12px] group">
       {profile?.picture && (
-        <img src={profile.picture} alt="" className="w-5 h-5 rounded-sm object-cover shrink-0" />
+        <img src={profile.picture} alt={`${name}'s avatar`} className="w-5 h-5 rounded-sm object-cover shrink-0" />
       )}
       <span className="text-text truncate flex-1">{name}</span>
       <button