fix(#94): replace text-white with text-primary-fg on all primary buttons

In dark mode --c-primary switches from navy (#012851) to mint (#a1dcd8).
Buttons using bg-primary+text-white showed white text on mint at 1.4:1
contrast — invisible. bg-brand-navy buttons were also invisible (navy on
near-black canvas, 1.3:1).

Replaced in 28 components app-wide:
- bg-primary ... text-white → text-primary-fg
- hover:bg-primary hover:text-white → hover:text-primary-fg
- bg-brand-navy ... text-white + hover:bg-brand-navy/90 →
  bg-primary ... text-primary-fg + hover:bg-primary/90

Light mode is unchanged: primary-fg = white in light mode.
Dark mode: primary-fg = navy (#012851) on mint bg = readable.

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

frontend/src/routes/admin/UsersTab.svelte

@@ -20,7 +20,7 @@ let {
 		<h2 class="text-lg font-bold text-ink-2">{m.admin_section_users()}</h2>
 		<a
 			href="/admin/users/new"
-			class="inline-flex items-center gap-1 rounded-sm bg-primary px-4 py-2 font-sans text-xs font-bold tracking-widest text-white uppercase transition-opacity hover:opacity-80"
+			class="inline-flex items-center gap-1 rounded-sm bg-primary px-4 py-2 font-sans text-xs font-bold tracking-widest text-primary-fg uppercase transition-opacity hover:opacity-80"
 		>
 			<svg class="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
 				<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4" />