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/components/editor/Toolbar.tsx

@@ -54,8 +54,8 @@ function BtnTooltip({ text, shortCode }: { text: string; shortCode?: string }) {
   );
 }
 
-type MarkButtonProps = { format: MarkType; icon: IconSrc; tooltip: ReactNode };
-export function MarkButton({ format, icon, tooltip }: MarkButtonProps) {
+type MarkButtonProps = { format: MarkType; icon: IconSrc; tooltip: ReactNode; label?: string };
+export function MarkButton({ format, icon, tooltip, label }: MarkButtonProps) {
   const editor = useSlate();
   const disableInline = isBlockActive(editor, BlockType.CodeBlock);
 
@@ -76,6 +76,7 @@ export function MarkButton({ format, icon, tooltip }: MarkButtonProps) {
           variant="SurfaceVariant"
           onClick={handleClick}
           aria-pressed={isMarkActive(editor, format)}
+          aria-label={label}
           size="400"
           radii="300"
           disabled={disableInline}
@@ -91,8 +92,9 @@ type BlockButtonProps = {
   format: BlockType;
   icon: IconSrc;
   tooltip: ReactNode;
+  label?: string;
 };
-export function BlockButton({ format, icon, tooltip }: BlockButtonProps) {
+export function BlockButton({ format, icon, tooltip, label }: BlockButtonProps) {
   const editor = useSlate();
 
   const handleClick = () => {
@@ -108,6 +110,7 @@ export function BlockButton({ format, icon, tooltip }: BlockButtonProps) {
           variant="SurfaceVariant"
           onClick={handleClick}
           aria-pressed={isBlockActive(editor, format)}
+          aria-label={label}
           size="400"
           radii="300"
         >
@@ -165,6 +168,7 @@ export function HeadingBlockButton() {
                   <IconButton
                     ref={triggerRef}
                     onClick={() => handleMenuSelect(1)}
+                    aria-label="Heading 1"
                     size="400"
                     radii="300"
                   >
@@ -180,6 +184,7 @@ export function HeadingBlockButton() {
                   <IconButton
                     ref={triggerRef}
                     onClick={() => handleMenuSelect(2)}
+                    aria-label="Heading 2"
                     size="400"
                     radii="300"
                   >
@@ -195,6 +200,7 @@ export function HeadingBlockButton() {
                   <IconButton
                     ref={triggerRef}
                     onClick={() => handleMenuSelect(3)}
+                    aria-label="Heading 3"
                     size="400"
                     radii="300"
                   >
@@ -271,32 +277,44 @@ export function Toolbar() {
               <MarkButton
                 format={MarkType.Bold}
                 icon={Icons.Bold}
-                tooltip={<BtnTooltip text="Bold" shortCode={`${modKey} + B`} />}
+                tooltip={<BtnTooltip text="Bold" shortCode={`${modKey} + B`}
+                label="Bold"
+              />}
               />
               <MarkButton
                 format={MarkType.Italic}
                 icon={Icons.Italic}
-                tooltip={<BtnTooltip text="Italic" shortCode={`${modKey} + I`} />}
+                tooltip={<BtnTooltip text="Italic" shortCode={`${modKey} + I`}
+                label="Italic"
+              />}
               />
               <MarkButton
                 format={MarkType.Underline}
                 icon={Icons.Underline}
-                tooltip={<BtnTooltip text="Underline" shortCode={`${modKey} + U`} />}
+                tooltip={<BtnTooltip text="Underline" shortCode={`${modKey} + U`}
+                label="Underline"
+              />}
               />
               <MarkButton
                 format={MarkType.StrikeThrough}
                 icon={Icons.Strike}
-                tooltip={<BtnTooltip text="Strike Through" shortCode={`${modKey} + S`} />}
+                tooltip={<BtnTooltip text="Strike Through" shortCode={`${modKey} + S`}
+                label="Strikethrough"
+              />}
               />
               <MarkButton
                 format={MarkType.Code}
                 icon={Icons.Code}
-                tooltip={<BtnTooltip text="Inline Code" shortCode={`${modKey} + [`} />}
+                tooltip={<BtnTooltip text="Inline Code" shortCode={`${modKey} + [`}
+                label="Inline code"
+              />}
               />
               <MarkButton
                 format={MarkType.Spoiler}
                 icon={Icons.EyeBlind}
-                tooltip={<BtnTooltip text="Spoiler" shortCode={`${modKey} + H`} />}
+                tooltip={<BtnTooltip text="Spoiler" shortCode={`${modKey} + H`}
+                label="Spoiler"
+              />}
               />
             </Box>
             <Line variant="SurfaceVariant" direction="Vertical" style={{ height: toRem(12) }} />
@@ -305,22 +323,30 @@ export function Toolbar() {
             <BlockButton
               format={BlockType.BlockQuote}
               icon={Icons.BlockQuote}
-              tooltip={<BtnTooltip text="Block Quote" shortCode={`${modKey} + '`} />}
+              tooltip={<BtnTooltip text="Block Quote" shortCode={`${modKey} + '`}
+              label="Block quote"
+            />}
             />
             <BlockButton
               format={BlockType.CodeBlock}
               icon={Icons.BlockCode}
-              tooltip={<BtnTooltip text="Block Code" shortCode={`${modKey} + ;`} />}
+              tooltip={<BtnTooltip text="Block Code" shortCode={`${modKey} + ;`}
+              label="Code block"
+            />}
             />
             <BlockButton
               format={BlockType.OrderedList}
               icon={Icons.OrderList}
-              tooltip={<BtnTooltip text="Ordered List" shortCode={`${modKey} + 7`} />}
+              tooltip={<BtnTooltip text="Ordered List" shortCode={`${modKey} + 7`}
+              label="Ordered list"
+            />}
             />
             <BlockButton
               format={BlockType.UnorderedList}
               icon={Icons.UnorderList}
-              tooltip={<BtnTooltip text="Unordered List" shortCode={`${modKey} + 8`} />}
+              tooltip={<BtnTooltip text="Unordered List" shortCode={`${modKey} + 8`}
+              label="Unordered list"
+            />}
             />
             <HeadingBlockButton />
           </Box>