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/enrich/+page.svelte

@@ -47,7 +47,7 @@ function formatUploadDate(createdAt: string): string {
 		{#if count > 0}
 			<a
 				href="/enrich/{documents[0].id}"
-				class="bg-brand-navy px-5 py-2 font-sans text-xs font-bold tracking-widest text-white uppercase transition-colors hover:bg-brand-navy/90"
+				class="bg-primary px-5 py-2 font-sans text-xs font-bold tracking-widest text-primary-fg uppercase transition-colors hover:bg-primary/90"
 			>
 				{m.enrich_list_start()}
 			</a>

frontend/src/routes/enrich/[id]/+page.svelte

@@ -156,7 +156,7 @@ let selectedReceivers = $state(untrack(() => doc.receivers ?? []));
 						type="submit"
 						form="save-form"
 						formaction="?/saveAndReview"
-						class="rounded-sm bg-brand-navy px-5 py-2 font-sans text-xs font-bold tracking-widest text-white uppercase transition-colors hover:bg-brand-navy/90"
+						class="rounded-sm bg-primary px-5 py-2 font-sans text-xs font-bold tracking-widest text-primary-fg uppercase transition-colors hover:bg-primary/90"
 					>
 						{m.btn_save_and_mark_reviewed()}
 					</button>

frontend/src/routes/enrich/done/+page.svelte

@@ -24,7 +24,7 @@ import { m } from '$lib/paraglide/messages.js';
 		<div class="mt-8 flex flex-col items-center gap-4">
 			<a
 				href="/"
-				class="bg-brand-navy px-6 py-2 font-sans text-xs font-bold tracking-widest text-white uppercase transition-colors hover:bg-brand-navy/90"
+				class="bg-primary px-6 py-2 font-sans text-xs font-bold tracking-widest text-primary-fg uppercase transition-colors hover:bg-primary/90"
 			>
 				{m.btn_back_to_overview()}
 			</a>