Assistant messages could not display inline images or attachments on the
mobile client. The markdown renderer had no image block, and
AssistantBubble only rendered MarkdownText + timestamp, ignoring
message.attachments (only UserBubble did).
- Add MdBlock.Image + standalone  parsing in MarkdownText.
Accepts http(s) gateway URLs and data:image/...;base64 inline media.
- Render message.attachments inside AssistantBubble (mirrors UserBubble)
via InlineAttachment / AsyncImage.
- Add parser unit tests for image blocks (url, data: URL, inline-in-paragraph).
Local verification: ./gradlew ktlintCheck compileDebugKotlin assembleDebug
all green; app-debug.apk produced.
Fixes feature gap where agent-delivered media (e.g. MEDIA:/ images
sent to chat) was invisible in the Android app.
Co-authored-by: M57 <hy4ri@users.noreply.github.com>