Add glass UI with backdrop blur and fix primary button colors

Apply glassmorphism to sidebar, cards, tables, modals, dropdowns,
and dialogs with semi-transparent backgrounds, backdrop-blur, and
white/15 borders. Add subtle gradient body background for the blur
to work against. Add dedicated btn-primary color tokens so primary
buttons stay bold in dark mode instead of using the brightened text
palette. Use shadow-sm to avoid heavy halos in light mode.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

resources/scripts/admin/layouts/partials/TheSiteSidebar.vue

@@ -124,8 +124,8 @@
       h-screen
       pb-32
       overflow-y-auto
-      bg-surface
-      border-r border-line-default border-solid
+      bg-surface/80 backdrop-blur-xl
+      border-r border-white/10
       xl:w-64
       md:fixed md:flex md:flex-col md:inset-y-0
       pt-16