feat(messaging-ui): mail attachment download + scrollable reader (0.1.1)

- Attachment chips in the mail reader are now clickable: new InboxAdapter
  downloadAttachment() resolves a short-lived URL, opened in a new tab.
- Mail reader scrolls again: convert the .miu-detail/.miu-mail height:100%
  chain to flex fill so a long thread scrolls within the pane instead of
  being clipped. MockInboxAdapter implements download. Bump to 0.1.1.

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

packages/iios-messaging-ui/src/adapters/mock-inbox.ts

@@ -88,6 +88,11 @@ export class MockInboxAdapter implements InboxAdapter {
     return { contentRef: `mock/${this.seq++}`, mimeType: file.type || 'application/octet-stream', sizeBytes: file.size, filename: file.name };
   }
 
+  async downloadAttachment(attachment: MailAttachment): Promise<string> {
+    // Demo: no real bytes — hand back a data URL so the click resolves without a network call.
+    return `data:${attachment.mimeType};base64,`;
+  }
+
   async directory(): Promise<MailPerson[]> {
     return [...PEOPLE];
   }