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/admin/SystemTab.svelte

@@ -41,7 +41,7 @@ async function backfillFileHashes() {
 	<button
 		onclick={backfillVersions}
 		disabled={backfillLoading}
-		class="rounded bg-primary px-6 py-2 text-sm font-bold text-white uppercase transition hover:bg-accent hover:text-ink disabled:cursor-not-allowed disabled:opacity-50"
+		class="rounded bg-primary px-6 py-2 text-sm font-bold text-primary-fg uppercase transition hover:bg-accent hover:text-ink disabled:cursor-not-allowed disabled:opacity-50"
 	>
 		{backfillLoading ? '…' : m.admin_system_backfill_btn()}
 	</button>
@@ -60,7 +60,7 @@ async function backfillFileHashes() {
 	<button
 		onclick={backfillFileHashes}
 		disabled={backfillHashesLoading}
-		class="rounded bg-primary px-6 py-2 text-sm font-bold text-white uppercase transition hover:bg-accent hover:text-ink disabled:cursor-not-allowed disabled:opacity-50"
+		class="rounded bg-primary px-6 py-2 text-sm font-bold text-primary-fg uppercase transition hover:bg-accent hover:text-ink disabled:cursor-not-allowed disabled:opacity-50"
 	>
 		{backfillHashesLoading ? '…' : m.admin_system_backfill_hashes_btn()}
 	</button>