Implement Massive Code Reduction Across Repo

Identify an opportunity to massively reduce the amount of code (lines of code) in any part of the repo: - Similar functionality that can be unified and reused - Code that has been valuable but no more - Front end component or group of them duplicates another component functionality. Use the most modern and nice looking - Move to DBOS and get rid of supporting functionality relying on DBOS - etc. Implement it

Net −118 LOC: unified mention/reaction feeds (−94) and 4 submit handlers moved to useSubmit (−24). PR #5279 merged; Jaah Deploy succeeded. Full gate green except org-switcher AC3 e2e, which fails identically on develop. Deferred: New-group org preselect (task creation 403), legacy cron mirroring kept (still live).