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/lib/components/PanelMetadata.svelte

@@ -100,7 +100,7 @@ let { doc }: { doc: Doc } = $props();
 							{#each doc.tags as tag (tag.id)}
 								<a
 									href="/?tag={encodeURIComponent(tag.name)}"
-									class="inline-flex items-center rounded bg-muted px-2 py-0.5 text-xs font-bold tracking-wide text-ink uppercase transition-colors hover:bg-primary hover:text-white"
+									class="inline-flex items-center rounded bg-muted px-2 py-0.5 text-xs font-bold tracking-wide text-ink uppercase transition-colors hover:bg-primary hover:text-primary-fg"
 									title={m.doc_tag_filter_title({ name: tag.name })}
 								>
 									{tag.name}
@@ -131,7 +131,7 @@ let { doc }: { doc: Doc } = $props();
 				>
 					<div class="flex items-center gap-3">
 						<div
-							class="flex h-8 w-8 items-center justify-center rounded-full bg-primary font-serif text-sm text-white"
+							class="flex h-8 w-8 items-center justify-center rounded-full bg-primary font-serif text-sm text-primary-fg"
 						>
 							{doc.sender.firstName[0]}{doc.sender.lastName[0]}
 						</div>