feat(desktop): embed linked buzz messages

[tellaho]

Category: new-feature
User Impact: Desktop users can now recognize and open linked Buzz messages from a rich inline preview without losing their place.

Problem: Bare Buzz message permalinks only appeared as compact pills, hiding the quoted context until opened. They also needed a strict disclosure boundary for private, missing, or mismatched sources.

Solution: Bare permalinks now append a bounded, stable-height preview with the source author, avatar, channel, and excerpt only after channel readability and event-channel integrity checks pass. Unreadable or unresolved sources render a neutral non-leaking state, while labeled links remain unchanged and excerpts never recursively unfurl.

Originating Buzz channel: 840b8580-b6f5-4cc5-90ff-6dd78d299ae1

File changes

desktop/src/features/messages/lib/messageEmbed.ts
Extracts bare message permalinks, centralizes channel readability and event-integrity checks, and bounds preview excerpts.

desktop/src/features/messages/lib/messageEmbed.test.mjs
Covers extraction, labeled/code exclusions, open and joined-private readability, mismatched source rejection, and excerpt bounds.

desktop/src/shared/ui/markdown/MessageEmbed.tsx
Fetches and renders the privacy-gated message preview with stable loading and neutral unavailable states.

desktop/src/shared/ui/markdown/MessageEmbedList.tsx
Resolves message embeds outside the cached Markdown node tree and appends them with existing attachment layout conventions.

desktop/src/shared/ui/markdown/MessageLinkPill.tsx
Prevents inaccessible links from exposing channel names or offering unsafe navigation.

desktop/src/shared/ui/markdown.tsx
Integrates the message embed list alongside existing rich preview attachments.

desktop/tests/e2e/navigation.spec.ts
Verifies a resolved embed shows source context and opens the linked thread.

Reproduction steps

1. Open a Desktop channel containing a readable source message.
2. Send a new message containing its bare buzz://message?... permalink.
3. Confirm the original link remains visible and a bordered preview shows the author, avatar, channel, and bounded source text.
4. Click the preview and confirm Desktop opens the linked message/thread.
5. Paste a link to an unreadable, missing, or channel-mismatched source and confirm only the neutral private/unavailable state appears.
6. Paste an intentionally labeled Markdown link and confirm it remains a label without a rich embed.

[Chessing234]

love the embeds. one thing — make sure private/missing messages stay as the plain pill and never fetch content the viewer isnt allowed to see