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/sidebar/AccountSwitcher.tsx

@@ -10,7 +10,7 @@ function Avatar({ account, size = "w-6 h-6", textSize = "text-[10px]" }: { accou
     return (
       <img
         src={account.picture}
-        alt=""
+        alt={`${account.name || "Account"} avatar`}
         className={`${size} rounded-sm object-cover shrink-0`}
         onError={(e) => {
           (e.target as HTMLImageElement).style.display = "none";