Identify and implement top five code reduction

Identify top 5 opportunities 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 - A front end component or a group of them duplicate another component/page functionality. Use the most modern and nice looking - etc. Now implement them :)

Top-5 reduction shipped and merged (PR #6957 → 84efcb8f): web reads onto shared hooks, URL updaters onto useUrlFilters, one SecretOnceDialog, closed-window API shims (+ re-backfill migration) and jaahd one-shots retired; net −339 lines. Gates green, two review lanes PASS, visual check done (filters and email sections render clean). Measured: no massive cut remains after three prior passes. Deploy run 40277487912 failed on a stale self-hosted slot workspace, not the diff; filed drafts 10185 (slot workspace) and 10173 (rerun policy). Memory: used mem-25697; proposed none.