a11y: give every icon-only sidebar avatar button and the message sender avatar an accessible name (#185)

axe-core flagged 18 'button-name' criticals on the room view: the space
tabs, Home/Direct/Inbox/Search/Saved/Explore/Add Space/User Settings/
Unverified sidebar buttons, and each message's avatar button had no text
for screen readers. Labels mirror the existing tooltips; message avatars
read '<name>, open profile'. Room view now has zero button-name findings.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/pages/client/sidebar/SettingsTab.tsx

@@ -184,7 +184,12 @@ export function SettingsTab() {
     <SidebarItem active={settingsOpen}>
       <SidebarItemTooltip tooltip="User Settings">
         {(triggerRef) => (
-          <SidebarAvatar as="button" ref={triggerRef} onClick={() => setSettingsOpen(true)}>
+          <SidebarAvatar
+            as="button"
+            ref={triggerRef}
+            aria-label="User Settings"
+            onClick={() => setSettingsOpen(true)}
+          >
             <UserAvatar
               userId={userId}
               src={avatarUrl}