theme auth/setup pages and unify resets

Phase 7: add LoadTheme to base :browser pipeline so auth and setup
pages get theme settings. Update root.html.heex with .themed wrapper,
font preloads, layer declaration, and generated CSS injection.
Remove old data-theme JS toggle script.

Phase 8: upgrade admin/reset.css to a proper @layer reset matching
the shop reset structure. Remove dead theme toggle CSS rules.

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

assets/css/admin/components.css

@@ -632,15 +632,6 @@
   &.active .admin-swap-off { display: none; }
 }
 
-/* ── Theme toggle ── */
-
-.theme-toggle-indicator {
-  transition: left 150ms cubic-bezier(0.4, 0, 0.2, 1);
-}
-
-[data-theme="light"] .theme-toggle-indicator { left: 33.333333%; }
-[data-theme="dark"] .theme-toggle-indicator { left: 66.666667%; }
-
 /* ── Dashboard stats grid ── */
 
 .admin-stats-grid {