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/devices/DeviceTile.tsx

@@ -292,6 +292,8 @@ export function DeviceTile({
             outlined={deleted}
             radii="300"
             onClick={() => setDetails(!details)}
+            aria-label={details ? 'Collapse device details' : 'Expand device details'}
+            aria-expanded={details}
           >
             <Icon size="50" src={details ? Icons.ChevronBottom : Icons.ChevronRight} />
           </IconButton>

src/app/features/settings/devices/Devices.tsx

@@ -74,7 +74,7 @@ export function Devices({ requestClose }: DevicesProps) {
             </Text>
           </Box>
           <Box shrink="No">
-            <IconButton onClick={requestClose} variant="Surface">
+            <IconButton onClick={requestClose} variant="Surface" aria-label="Close">
               <Icon src={Icons.Cross} />
             </IconButton>
           </Box>

src/app/features/settings/devices/Verification.tsx

@@ -293,6 +293,7 @@ export function DeviceVerificationOptions() {
         size="300"
         radii="300"
         onClick={handleMenu}
+        aria-label="Verification options"
       >
         <Icon size="100" src={Icons.VerticalDots} />
       </IconButton>