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/notifications/KeywordMessages.tsx

@@ -80,6 +80,7 @@ function KeywordInput() {
                 size="300"
                 radii="300"
                 variant="Secondary"
+                aria-label="Clear keyword input"
               >
                 <Icon src={Icons.Cross} size="100" />
               </IconButton>
@@ -118,7 +119,7 @@ function KeywordCross({ pushRule }: PushRulesProps) {
 
   const removing = removeState.status === AsyncStatus.Loading;
   return (
-    <IconButton onClick={remove} size="300" radii="Pill" variant="Secondary" disabled={removing}>
+    <IconButton onClick={remove} size="300" radii="Pill" variant="Secondary" disabled={removing} aria-label="Remove keyword">
       {removing ? <Spinner size="100" /> : <Icon src={Icons.Cross} size="100" />}
     </IconButton>
   );