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/+page.svelte

@@ -20,28 +20,28 @@ let activeTab = $state('users');
 			<button
 				class="rounded-md px-2 py-2 text-sm font-bold tracking-wide uppercase transition sm:px-4 {activeTab ===
 				'users'
-					? 'bg-primary text-white'
+					? 'bg-primary text-primary-fg'
 					: 'text-ink-2 hover:text-ink'}"
 				onclick={() => (activeTab = 'users')}>{m.admin_tab_users()}</button
 			>
 			<button
 				class="rounded-md px-2 py-2 text-sm font-bold tracking-wide uppercase transition sm:px-4 {activeTab ===
 				'groups'
-					? 'bg-primary text-white'
+					? 'bg-primary text-primary-fg'
 					: 'text-ink-2 hover:text-ink'}"
 				onclick={() => (activeTab = 'groups')}>{m.admin_tab_groups()}</button
 			>
 			<button
 				class="rounded-md px-2 py-2 text-sm font-bold tracking-wide uppercase transition sm:px-4 {activeTab ===
 				'tags'
-					? 'bg-primary text-white'
+					? 'bg-primary text-primary-fg'
 					: 'text-ink-2 hover:text-ink'}"
 				onclick={() => (activeTab = 'tags')}>{m.admin_tab_tags()}</button
 			>
 			<button
 				class="rounded-md px-2 py-2 text-sm font-bold tracking-wide uppercase transition sm:px-4 {activeTab ===
 				'system'
-					? 'bg-primary text-white'
+					? 'bg-primary text-primary-fg'
 					: 'text-ink-2 hover:text-ink'}"
 				onclick={() => (activeTab = 'system')}>{m.admin_tab_system()}</button
 			>