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>