fix(a11y): add missing aria-labels to message buttons

- FallbackContent: '(edited)' button now has aria-label='View edit history'
- Reply: ThreadIndicator as=button gets aria-label='View thread'
- Reply: ReplyLayout as=button gets aria-label='Jump to original message'

Addresses WCAG 2.1 SC 4.1.2 (Name, Role, Value) for icon-only buttons
and polymorphic div-as-button patterns in the message timeline.

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

src/app/components/message/content/FallbackContent.tsx

@@ -75,6 +75,7 @@ export const MessageEditedContent = as<
       <button
         type="button"
         onClick={onEditHistoryClick}
+        aria-label="View edit history"
         style={{ cursor: 'pointer', background: 'none', border: 'none', padding: 0 }}
       >
         <Text as="span" size="T200" priority="300">