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/url-preview/UrlPreviewCard.tsx

@@ -182,6 +182,7 @@ export const UrlPreviewHolder = as<'div'>(({ children, ...props }, ref) => {
               <div className={css.UrlPreviewHolderGradient({ position: 'Left' })} />
               <IconButton
                 className={css.UrlPreviewHolderBtn({ position: 'Left' })}
+                aria-label="Previous preview"
                 variant="Secondary"
                 radii="Pill"
                 size="300"
@@ -200,6 +201,7 @@ export const UrlPreviewHolder = as<'div'>(({ children, ...props }, ref) => {
                 <div className={css.UrlPreviewHolderGradient({ position: 'Right' })} />
                 <IconButton
                   className={css.UrlPreviewHolderBtn({ position: 'Right' })}
+                  aria-label="Next preview"
                   variant="Primary"
                   radii="Pill"
                   size="300"