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 :)

Shipped PR #7950 (queued to the merge lane): -2,002 net lines. 26 post-baseline migrations folded into baseline 202611031500 with catalogue and row-set identity proved; credential testers, one console pager, three or_404 sites and three UuidPkMixin ids folded. Quick gate green; mechanical censuses found no other large remnant. Visual check done: shared pager footer renders at desktop and phone. Memory: used mem-25288, mem-22590; proposed 1 recipe (mem-37210).