feat(a11y): comprehensive icon button label sweep — 60+ remaining buttons labeled

C-1 complete sweep across all components and features:
- Call controls: mic mute/unmute, deafen/undeafen, video, screenshare, chat
- RoomInput: dismiss reply, attach file, sticker, emoji, GIF, location, toolbar
- Media viewers: close in image/pdf/text viewers and editors
- Settings dialogs: close buttons in all room/space/common settings panels
- Lobby: back, toggle member list, scroll to top, pack add/remove
- Auth: server picker, UIA flow cancel
- Upload cards: cancel uploads
- URL preview: prev/next buttons
- Members drawer: close + scroll to top
- RoomViewHeader: back, start call, toggle member list

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

src/app/components/image-viewer/ImageViewer.tsx

@@ -33,7 +33,7 @@ export const ImageViewer = as<'div', ImageViewerProps>(
       >
         <Header className={css.ImageViewerHeader} size="400">
           <Box grow="Yes" alignItems="Center" gap="200">
-            <IconButton size="300" radii="300" onClick={requestClose}>
+            <IconButton size="300" radii="300" onClick={requestClose} aria-label="Close">
               <Icon size="50" src={Icons.ArrowLeft} />
             </IconButton>
             <Text size="T300" truncate>