Fix device verification UX: show request card, enable cross-user SAS

- RenderMessageContent: add case for m.key.verification.request msgtype
  so it renders an informational card instead of "Unsupported message"
- MsgTypeRenderers/FallbackContent: add VerificationRequestContent and
  MessageVerificationRequestContent components (lock icon + instructional text)
- DeviceVerification: remove isSelfVerification guard from
  ReceiveSelfDeviceVerification so cross-user verification requests also
  trigger the SAS emoji dialog (was silently dropped before)

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

src/app/components/DeviceVerification.tsx

@@ -316,9 +316,5 @@ export function ReceiveSelfDeviceVerification() {
 
   if (!request) return null;
 
-  if (!request.isSelfVerification) {
-    return null;
-  }
-
   return <DeviceVerification request={request} onExit={handleExit} />;
 }