Reduce repository code volume by implementing top

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 :)

Measured first (jscpd 0.15%, test-value detectors burned out): no massive cut remains; shipped the five largest safe folds — or_404 over 81 route 404 sites, notify Leg, one node-registration payload home, PageNav for the exclusions pager, EmptyState in the command center — net −115 lines/73 files. Quick gate, lint ratchet, vitest 1,419 and API route tests 1,555 green; review lanes pass; visual check done (command center and Audit › Exclusions clean at 1440/448px). PR #7556 queued to the merge lane. Deferred: legacy offset pager onto Page[T]; ADR-level levers named in the report. Memory: used mem-32217, mem-31277; proposed 0.