fix(desktop): flush stale channel autocomplete before Enter

[BrianInAz]

Summary

• resolve pending channel autocomplete from the editor's current text and cursor before Enter or Tab can commit
• close stale autocomplete and hand inactive Enter back to the existing submit path in both message and forum composers
• make the empty-edit regression deterministic and retain the corrected 10-second post-Enter assertions for loaded CI shards

Root cause

useChannelLinks retained the previous channel query and suggestion list during its 120 ms debounce. If an editor containing #general was cleared and Enter arrived before that debounce fired, the stale autocomplete consumed Enter and reinserted #general; the empty-edit delete handler never received empty content.

This is pre-existing upstream behavior. The production hook was identical to upstream before this hotfix, and the intervening fork commits did not touch the failure path.

TDD evidence

Red on ce5acf44:

• the pure resolver test failed because no fresh-state resolver existed
• the deterministic Playwright spec failed both empty-edit cases while the non-empty edit passed
• failure screenshots retained edit mode with a reinserted #general and no delete dialog after 10 seconds

Green on da4b5cb47:

• resolver unit tests: 5/5
• focused empty-edit Playwright spec: 3/3
• --repeat-each=10 stress proof: 30/30
• Desktop helper suite: 3,959/3,959
• desktop-check, Desktop build/tests, Tauri checks/tests, web checks/build, mobile checks/tests, and complete just ci: passed
• final full CI ran with Cargo offline after fetching only lockfile-pinned dependencies
• Cargo.lock, desktop/src-tauri/Cargo.lock, mobile/pubspec.lock, and pnpm-lock.yaml: unchanged
• git diff --check and scoped secret scan: clean

Tracking

• Beads: ios-buzz-59e.16

Rollback

Revert the squash commit. There is no protocol, persistence, relay, CLI, mobile, or migration impact.

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