fix(desktop): restore scoped typing indicators in forum threads

[context-down]

Summary

Restore existing typing indicators in Forum threads. The desktop currently skips Forum typing subscriptions and does not render their status.

• Subscribe to existing typing events for Forums.
• Scope nested-comment typing to its Forum root post; preserve Stream immediate-parent scoping.
• Render the existing accessible typing row above the selected Forum reply composer, with profile name resolution.
• Preserve TTL expiry and post-completion suppression, including Forum completion kinds when supplied to the hook.
• Add six production-hook regressions and a mock-bridge browser workflow for selected-post visibility, other-post exclusion, and expiry.

This branch contains only the eight-file desktop repair, ported to upstream deda09c18. It does not include the older ACP/relay work from the original local branch. Merge conflict resolution preserves upstream mention-copy support, post IDs, and existing smoke tests.

Validation

• PASS: 16 focused typing/channel-activity tests.
• PASS: focused Biome check for seven changed files; git diff --check.
• PASS: frontend E2E build (TypeScript and Vite).
• PASS: full desktop formatting/copy checks (desktop-check).
• PASS: production frontend build, including both protected-feature artifact variants, through the native demo-build recipe.
• PASS: all six Apple Silicon release sidecars compiled and bundled.
• INCOMPLETE: broad desktop tests were interrupted earlier, with no completion result; not claimed green.
• PASS: isolated Chrome mock-bridge workflow (11.2 seconds; 13.8 seconds total): selected-post typing visible, other-post excluded, stopped heartbeat expired. Rerun allowed 60 seconds for total setup/workflow while preserving the original 11-second expiry assertion. Prior concurrent run exhausted the default total deadline. Installed Chrome used with video disabled because Playwright browser/ffmpeg cache is absent; no assertions removed.
• just ci was initially blocked by Xcode setup. The user has since completed that setup; xcodebuild first-launch readiness now passes. Full repository CI has not been rerun to completion.
• BLOCKED: isolated native desktop app build stopped at its disk-space reserve after 21m33s (host free space dropped below 4 GiB). Completed sidecars and production frontend are retained; no installable app was produced. No release, installed-app replacement, or deletion performed.

Desktop delivery

Draft pending complete validation and upstream review. Source publication is not a native app installation. Maintainer merge plus the upstream Signed macOS Canary workflow (main-only), or a normal desktop release, is required for upstream-signed native-client testing. The separately identified local unsigned demo-build path is implemented by the existing repository recipe but is currently blocked by host free space. No release/tag/updater promotion requested or performed. Actual relay heartbeat receipt and native indicator appearance/disappearance remain acceptance checks.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is deda09c18c78d48847b032557c331f28faf64ee8...74acc6a10dd129f89648916a0a2f381ce5950207.
A new review must complete for this exact range. When manual authorization
is required, a Block organization member must comment exactly
@buzz-security-review 74acc6a10dd129f89648916a0a2f381ce5950207 to authorize a new review.
Any previous review applies only to its recorded range.