fix(desktop): forum deep links open the target post, not the post list

[BachirSaaS]

Summary

A buzz://message deep link into a forum channel navigated to the base channel route, which passes selectedPostId={null} — the forum view rendered the post list and the target post was never opened (#7315). In-app navigation worked because it targets the /channels/$channelId/posts/$postId subroute.

When the active channel is a forum and a target message arrives on the base route, redirect (replace) to the post subroute:

• post id comes from the explicit thread root when the link carries one
• otherwise it is derived from the fetched target event's own thread reference (its root is the post), with the target passed as replyId so the thread scrolls to it
• keyed per channel+target so re-renders never double-fire

Testing

pnpm exec tsc -b clean; deep-link paths covered by the existing useMessageDeepLinks routing (unchanged — the fix is downstream of it, where channel type is known).

Fixes #7315

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

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[BachirSaaS]

Closing — we'll maintain these fixes in our fork instead. Thanks for the look.