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/user-profile/PowerChip.tsx

@@ -70,7 +70,7 @@ function SelfDemoteAlert({ power, onCancel, onChange }: SelfDemoteAlertProps) {
               <Box grow="Yes">
                 <Text size="H4">Self Demotion</Text>
               </Box>
-              <IconButton size="300" onClick={onCancel} radii="300">
+              <IconButton size="300" onClick={onCancel} radii="300" aria-label="Cancel">
                 <Icon src={Icons.Cross} />
               </IconButton>
             </Header>