Reduce repository code by implementing five major

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 :)

Built the five best genuine code reductions (HTTPException collapse, useSubmit migrations, one home for GitHub reads, shared guard skeleton, helper single-homing): 70 files, net −303 lines, no behaviour change. Review lanes pass; PR #7538 queued to the merge lane. "Massive" needs a platform-drop decision (Windows/macOS), deferred. Visual check done: dialog, legal form and project page render cleanly. Memory: used mem-31667, mem-31277, mem-31548; proposed none.