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/emojis-stickers/GlobalPacks.tsx

@@ -373,6 +373,7 @@ export function GlobalPacks({ onViewPack }: GlobalPacksProps) {
                   variant="Critical"
                   onClick={() => handleUndoRemove(address)}
                   disabled={applyingChanges}
+                  aria-label="Undo remove pack"
                 >
                   <Icon src={Icons.Plus} size="100" />
                 </IconButton>
@@ -383,6 +384,7 @@ export function GlobalPacks({ onViewPack }: GlobalPacksProps) {
                   variant="Secondary"
                   onClick={() => handleRemove(address)}
                   disabled={applyingChanges}
+                  aria-label="Remove pack"
                 >
                   <Icon src={Icons.Cross} size="100" />
                 </IconButton>