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/users/[id]/+page.svelte

@@ -45,13 +45,13 @@ const initials = $derived.by(() => {
 			<div class="mb-5 flex justify-center">
 				{#if initials}
 					<div
-						class="flex h-16 w-16 items-center justify-center rounded-full bg-primary text-white"
+						class="flex h-16 w-16 items-center justify-center rounded-full bg-primary text-primary-fg"
 					>
 						<span class="font-serif text-xl font-bold">{initials}</span>
 					</div>
 				{:else}
 					<div
-						class="flex h-16 w-16 items-center justify-center rounded-full bg-primary text-white"
+						class="flex h-16 w-16 items-center justify-center rounded-full bg-primary text-primary-fg"
 					>
 						<svg
 							class="h-8 w-8"