feat(a11y): message semantics for screen readers (P3-4)

- Each message is role="article"; collapsed messages (consecutive from one
  sender) now carry an aria-label with sender + time — previously a screen
  reader heard only the body with no attribution (the biggest a11y gap).
  Pure messageAriaLabel() reuses the existing time utils (+3 tests).
- Editing a message announces "Editing message from <sender>" (ariaLabel
  threaded MessageEditor → CustomEditor; the main composer is unaffected).
- System emoji get role="img" + aria-label from the shortcode; custom
  emoticons always have an accessible name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/editor/Editor.tsx

@@ -66,6 +66,8 @@ type CustomEditorProps = {
   maxHeight?: string;
   editor: Editor;
   placeholder?: string;
+  /** Explicit accessible name for the textbox; falls back to the placeholder. */
+  ariaLabel?: string;
   onKeyDown?: KeyboardEventHandler;
   onKeyUp?: KeyboardEventHandler;
   onChange?: EditorChangeHandler;
@@ -82,6 +84,7 @@ export const CustomEditor = forwardRef<HTMLDivElement, CustomEditorProps>(
       maxHeight = '50vh',
       editor,
       placeholder,
+      ariaLabel,
       onKeyDown,
       onKeyUp,
       onChange,
@@ -139,7 +142,7 @@ export const CustomEditor = forwardRef<HTMLDivElement, CustomEditorProps>(
                 data-editable-name={editableName}
                 className={css.EditorTextarea}
                 placeholder={placeholder}
-                aria-label={placeholder ?? 'Message input'}
+                aria-label={ariaLabel ?? placeholder ?? 'Message input'}
                 aria-multiline="true"
                 renderPlaceholder={renderPlaceholder}
                 renderElement={renderElement}