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Admin section under /admin, post-login deep-link fix, and common error pages #322

Job	Run time
Python typecheck	14s
JS lint & format	16s
Python lint & format	16s
Perf guards (Playwright)	1m 35s
Python tests	3m 53s
JS tests (Vitest)	28s
JS build (Vite)	42s
Package build (release dry-run)	12s
File size (300-line cap)	18s
JS typecheck	40s
E2E smoke (Playwright)	2m 3s
PR checks	4s
	10m 41s