fix(upload): use border-ink/20 and primary color for drop zone visibility

In light mode, border-line-2 (#eeede8) was nearly invisible and
accent (#a1dcd8, mint) was too light for hover text. Switch to:
- border-ink/20 — navy-tinted dashed border, readable in both modes
- hover:border-primary / hover:text-primary — navy in light, mint in dark

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -298,8 +298,8 @@ $effect(() => {
 			role="button"
 			tabindex="0"
 			class="mb-4 flex cursor-pointer items-center justify-center gap-3 border border-dashed px-6 py-3 text-sm transition-colors duration-150 {isDragging
-				? 'border-accent bg-accent/5 text-accent'
-				: 'border-line-2 text-ink-3 hover:border-accent hover:text-accent'}"
+				? 'border-primary bg-accent-bg text-primary'
+				: 'border-ink/20 text-ink-3 hover:border-primary hover:text-primary'}"
 			ondragover={handleDragOver}
 			ondragleave={handleDragLeave}
 			ondrop={handleDrop}