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/account/Profile.tsx

@@ -185,7 +185,7 @@ function ProfileAvatar({ profile, userId }: ProfileProps) {
                 <Box grow="Yes">
                   <Text size="H4">Remove Avatar</Text>
                 </Box>
-                <IconButton size="300" onClick={() => setAlertRemove(false)} radii="300">
+                <IconButton size="300" onClick={() => setAlertRemove(false)} radii="300" aria-label="Cancel">
                   <Icon src={Icons.Cross} />
                 </IconButton>
               </Header>
@@ -278,6 +278,7 @@ function ProfileDisplayName({ profile, userId }: ProfileProps) {
                     size="300"
                     radii="300"
                     variant="Secondary"
+                    aria-label="Reset display name"
                   >
                     <Icon src={Icons.Cross} size="100" />
                   </IconButton>