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/RenderMessageContent.tsx

@@ -22,6 +22,7 @@ import {
   RenderBody,
   ThumbnailContent,
   UnsupportedContent,
+  VerificationRequestContent,
   VideoContent,
 } from './message';
 import { UrlPreviewCard, UrlPreviewHolder } from './url-preview';
@@ -264,5 +265,9 @@ export function RenderMessageContent({
     return <MBadEncrypted />;
   }
 
+  if (msgType === 'm.key.verification.request') {
+    return <VerificationRequestContent />;
+  }
+
   return <UnsupportedContent />;
 }