Identify and implement top 5 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 :)

Five code reductions folded (76 files, +830/−1025, net −195): one billing-details editor (legacy dialog deleted), one row-selection home, jaahd file_io/read_json_or, api like_pattern/get_or_404/count_by/iso_or_none, useSubmit at 14 sites. Quick gate + changed-area suites green; blind code and security lanes pass. PR #7044 queued in the merge lane. Rejected the largest find (contraction.py, ~1150 lines: prescribed form for the planned identity rev-2 contraction) and the NodeLink v1 / vocabulary shims; named as deferred. Visual check done on Nodes, Slots and the Disable confirm (billing surface needs Stripe, covered by tests). Memory: used mem-25288, mem-25697, mem-23434; proposed none.