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/UserMenu.svelte

@@ -33,7 +33,7 @@ function clickOutside(node: HTMLElement) {
 			aria-expanded={userMenuOpen}
 			aria-haspopup="true"
 			onclick={() => (userMenuOpen = !userMenuOpen)}
-			class="flex h-8 w-8 items-center justify-center rounded-full bg-primary font-sans text-xs font-bold text-white transition-opacity hover:opacity-80"
+			class="flex h-8 w-8 items-center justify-center rounded-full bg-primary font-sans text-xs font-bold text-primary-fg transition-opacity hover:opacity-80"
 		>
 			{userInitials}
 		</button>