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/CommentThread.svelte

@@ -188,7 +188,7 @@ onMount(() => {
 			></textarea>
 			<div class="flex items-center gap-3">
 				<button
-					class="rounded bg-primary px-3 py-1.5 font-sans text-xs font-medium text-white hover:bg-primary/80 disabled:opacity-40"
+					class="rounded bg-primary px-3 py-1.5 font-sans text-xs font-medium text-primary-fg hover:bg-primary/80 disabled:opacity-40"
 					disabled={posting}
 					onclick={() => saveEdit(comment.id)}
 				>
@@ -291,7 +291,7 @@ onMount(() => {
 					></textarea>
 					<div class="flex items-center gap-3">
 						<button
-							class="rounded bg-primary px-3 py-1.5 font-sans text-xs font-medium text-white hover:bg-primary/80 disabled:opacity-40"
+							class="rounded bg-primary px-3 py-1.5 font-sans text-xs font-medium text-primary-fg hover:bg-primary/80 disabled:opacity-40"
 							disabled={posting}
 							onclick={() => postReply(thread.id)}
 						>
@@ -321,7 +321,7 @@ onMount(() => {
 				></textarea>
 				<div>
 					<button
-						class="rounded bg-primary px-3 py-1.5 font-sans text-xs font-medium text-white hover:bg-primary/80 disabled:opacity-40"
+						class="rounded bg-primary px-3 py-1.5 font-sans text-xs font-medium text-primary-fg hover:bg-primary/80 disabled:opacity-40"
 						disabled={posting || !newText.trim()}
 						onclick={postComment}
 					>