feat(a11y,perf): comprehensive icon button labels, toolbar a11y, timeline binary search

A11y C-1: aria-label on 30+ remaining icon-only buttons across:
  - settings panels (close, reset, info, expand, remove, undo)
  - editor toolbar (bold, italic, underline, strike, code, spoiler,
    blockquote, code block, ordered/unordered list, headings 1-3)
  - auth stages (cancel buttons in SSO, Password stages)
  - device verification (cancel buttons)
  - password input (show/hide toggle with dynamic label)
  - event readers, account data editor close buttons
  - global emoji packs (add/remove buttons)
Perf-5: Replace O(N×T) getTimelineAndBaseIndex scan with precomputed binary
  search (timelineSegments useMemo) — O(log T) per visible message render

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

src/app/features/settings/Settings.tsx

@@ -141,7 +141,7 @@ export function Settings({ initialPage, requestClose }: SettingsProps) {
               </Box>
               <Box shrink="No">
                 {screenSize === ScreenSize.Mobile && (
-                  <IconButton onClick={requestClose} variant="Background">
+                  <IconButton onClick={requestClose} variant="Background" aria-label="Close settings">
                     <Icon src={Icons.Cross} />
                   </IconButton>
                 )}