fix(a11y): add aria-label to exit-formatting and pin-menu buttons (P3-4)

Two icon-adjacent buttons were missing descriptive labels: the
"Exit formatting" key-symbol button in Toolbar.tsx and the "Pinned
messages" pin icon in RoomViewHeader.tsx.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/components/editor/Toolbar.tsx

@@ -252,6 +252,7 @@ export function ExitFormatting({ tooltip }: ExitFormattingProps) {
           onClick={handleClick}
           size="400"
           radii="300"
+          aria-label="Exit formatting"
         >
           <Text size="B400">{`Exit ${KeySymbol.Hyper}`}</Text>
         </IconButton>

src/app/features/room/RoomViewHeader.tsx

@@ -652,6 +652,7 @@ export function RoomViewHeader({ callView }: { callView?: boolean }) {
                   style={{ position: 'relative' }}
                   onClick={handleOpenPinMenu}
                   ref={triggerRef}
+                  aria-label="Pinned messages"
                   aria-pressed={!!pinMenuAnchor}
                 >
                   {pinnedEvents.length > 0 && (