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/features/lobby/Lobby.tsx

@@ -446,6 +446,7 @@ export function Lobby() {
                   >
                     <IconButton
                       onClick={() => virtualizer.scrollToOffset(0)}
+                      aria-label="Scroll to top"
                       variant="SurfaceVariant"
                       radii="Pill"
                       outlined

src/app/features/lobby/LobbyHeader.tsx

@@ -165,7 +165,7 @@ export function LobbyHeader({ showProfile, powerLevels }: LobbyHeaderProps) {
             <Box shrink="No">
               <BackRouteHandler>
                 {(onBack) => (
-                  <IconButton fill="None" onClick={onBack}>
+                  <IconButton fill="None" onClick={onBack} aria-label="Back">
                     <Icon src={Icons.ArrowLeft} />
                   </IconButton>
                 )}
@@ -221,6 +221,7 @@ export function LobbyHeader({ showProfile, powerLevels }: LobbyHeaderProps) {
                 <IconButton
                   fill="None"
                   ref={triggerRef}
+                  aria-label="Toggle member list"
                   onClick={() => setPeopleDrawer((drawer) => !drawer)}
                 >
                   <Icon size="400" src={Icons.User} />