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/DocumentList.svelte

@@ -121,7 +121,7 @@ let {
 										{#each doc.tags as tag (tag.id)}
 											<button
 												type="button"
-												class="relative z-10 inline-flex cursor-pointer items-center rounded bg-muted px-2 py-1 text-[10px] font-bold tracking-widest text-ink uppercase transition-colors hover:bg-primary hover:text-white"
+												class="relative z-10 inline-flex cursor-pointer items-center rounded bg-muted px-2 py-1 text-[10px] font-bold tracking-widest text-ink uppercase transition-colors hover:bg-primary hover:text-primary-fg"
 												onclick={(e) => {
 													e.preventDefault();
 													e.stopPropagation();