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/inbox/adapter.ts

@@ -22,6 +22,10 @@ export interface InboxAdapter {
   /** Upload a file to storage, returning a reference to send with a reply/compose. */
   uploadAttachment?(file: File): Promise<MailAttachment>;
 
+  /** Resolve a short-lived URL to view/download an attachment. Absent => attachment chips are
+   *  shown but not clickable. */
+  downloadAttachment?(attachment: MailAttachment): Promise<string>;
+
   // ── Compose (optional) — absent hides the "New message" affordance ──
   /** People you can compose an in-app message to. */
   directory?(): Promise<MailPerson[]>;