fix visual regressions in admin CSS refactor

Five missing line-height values caused pixel shifts when Tailwind
utilities (text-sm, text-lg, text-xs) were replaced with semantic
classes that only set font-size. Also remove phantom padding-bottom
on .admin-header (the old pb-4 utility was never defined).

Fixes: .admin-header, .admin-header-subtitle, .admin-error,
.admin-brand, .admin-text-secondary

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/css-migration.md

@@ -1,6 +1,6 @@
 # Plan: CSS migration — Tailwind + DaisyUI to modern hand-written CSS
 
-Status: Complete (all phases 0-8)
+Status: Complete (all phases 0-8). Admin CSS semantic refactor in progress — see PROGRESS.md.
 
 ## Overview