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/room/RoomTimeline.tsx

@@ -546,6 +546,17 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
   const timelineRef = React.useRef(timeline);
   timelineRef.current = timeline;
   const eventsLength = getTimelinesEventsCount(timeline.linkedTimelines);
+
+  // Perf-5: precompute base offsets once per linkedTimelines change instead of O(N×T) scan
+  const timelineSegments = useMemo<Array<[number, number, EventTimeline]>>(() => {
+    let base = 0;
+    return timeline.linkedTimelines.map((t) => {
+      const len = t.getEvents().length;
+      const seg: [number, number, EventTimeline] = [base, len, t];
+      base += len;
+      return seg;
+    });
+  }, [timeline.linkedTimelines]);
   const liveTimelineLinked =
     timeline.linkedTimelines[timeline.linkedTimelines.length - 1] === getLiveTimeline(room);
   const canPaginateBack =
@@ -1809,7 +1820,20 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
   let newDivider = false;
   let dayDivider = false;
   const eventRenderer = (item: number) => {
-    const [eventTimeline, baseIndex] = getTimelineAndBaseIndex(timeline.linkedTimelines, item);
+    // Perf-5: O(T) → O(log T) via precomputed segments
+    let eventTimeline: EventTimeline | undefined;
+    let baseIndex = 0;
+    {
+      let lo = 0;
+      let hi = timelineSegments.length - 1;
+      while (lo <= hi) {
+        const mid = (lo + hi) >>> 1;
+        const [base, len] = timelineSegments[mid];
+        if (item < base) { hi = mid - 1; }
+        else if (item >= base + len) { lo = mid + 1; }
+        else { eventTimeline = timelineSegments[mid][2]; baseIndex = base; break; }
+      }
+    }
     if (!eventTimeline) return null;
     const timelineSet = eventTimeline?.getTimelineSet();
     const mEvent = getTimelineEvent(eventTimeline, getTimelineRelativeIndex(item, baseIndex));