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/persons/+page.svelte

@@ -107,7 +107,7 @@ function handleSearch() {
 						<!-- Avatar -->
 						<div class="flex-shrink-0">
 							<div
-								class="flex h-12 w-12 items-center justify-center rounded-full bg-primary font-serif text-lg text-white transition-colors group-hover:bg-accent group-hover:text-ink"
+								class="flex h-12 w-12 items-center justify-center rounded-full bg-primary font-serif text-lg text-primary-fg transition-colors group-hover:bg-accent group-hover:text-ink"
 							>
 								{person.firstName[0]}{person.lastName[0]}
 							</div>

frontend/src/routes/persons/[id]/PersonCard.svelte

@@ -143,7 +143,7 @@ $effect(() => {
 					<div class="flex gap-3">
 						<button
 							type="submit"
-							class="rounded bg-primary px-5 py-2 text-sm font-bold tracking-widest text-white uppercase transition-colors hover:bg-primary/80"
+							class="rounded bg-primary px-5 py-2 text-sm font-bold tracking-widest text-primary-fg uppercase transition-colors hover:bg-primary/80"
 						>
 							{m.btn_save()}
 						</button>

frontend/src/routes/persons/[id]/PersonDocumentList.svelte

@@ -44,7 +44,7 @@ const visibleDocuments = $derived(
 <div class="mb-10">
 	<div class="mb-6 flex items-center gap-3 border-b border-ink/10 pb-2">
 		<h2 class="font-serif text-xl text-ink">{heading}</h2>
-		<span class="rounded-full bg-primary px-2 py-1 text-xs font-bold text-white">
+		<span class="rounded-full bg-primary px-2 py-1 text-xs font-bold text-primary-fg">
 			{documents.length}
 		</span>
 		{#if yearRange}

frontend/src/routes/persons/new/+page.svelte

@@ -89,7 +89,7 @@ let { form } = $props();
 			</a>
 			<button
 				type="submit"
-				class="rounded bg-primary px-6 py-2 text-sm font-bold tracking-widest text-white uppercase transition-colors hover:bg-primary/80"
+				class="rounded bg-primary px-6 py-2 text-sm font-bold tracking-widest text-primary-fg uppercase transition-colors hover:bg-primary/80"
 			>
 				{m.btn_create()}
 			</button>