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/room/RoomViewHeader.tsx

@@ -340,6 +340,7 @@ function CallButton() {
             variant="Surface"
             fill="None"
             ref={triggerRef}
+            aria-label="Start call"
             onClick={handleOpenMenu}
             onContextMenu={(evt) => {
               evt.preventDefault();
@@ -456,7 +457,7 @@ export function RoomViewHeader({ callView }: { callView?: boolean }) {
           <BackRouteHandler>
             {(onBack) => (
               <Box shrink="No" alignItems="Center">
-                <IconButton fill="None" onClick={onBack}>
+                <IconButton fill="None" onClick={onBack} aria-label="Back">
                   <Icon src={Icons.ArrowLeft} />
                 </IconButton>
               </Box>
@@ -615,7 +616,7 @@ export function RoomViewHeader({ callView }: { callView?: boolean }) {
               }
             >
               {(triggerRef) => (
-                <IconButton fill="None" ref={triggerRef} onClick={handleMemberToggle}>
+                <IconButton fill="None" ref={triggerRef} onClick={handleMemberToggle} aria-label="Toggle member list">
                   <Icon size="400" src={Icons.User} />
                 </IconButton>
               )}