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/PanelHistory.svelte

@@ -328,7 +328,7 @@ $effect(() => {
 				<button
 					onclick={applyCompare}
 					disabled={!compareA || !compareB || compareA === compareB}
-					class="w-full rounded bg-primary px-3 py-1.5 font-sans text-xs font-medium text-white transition hover:bg-primary/80 disabled:cursor-not-allowed disabled:opacity-40"
+					class="w-full rounded bg-primary px-3 py-1.5 font-sans text-xs font-medium text-primary-fg transition hover:bg-primary/80 disabled:cursor-not-allowed disabled:opacity-40"
 				>
 					{m.history_compare_apply()}
 				</button>