fix(#98): make drop zone border and card borders visible in dark mode

- DropZone: raise border opacity from /20 to /30 for dashed drop zone
- layout.css: bump dark mode --c-line from #2e2e2e to #3d3d3d (was
  ~1.3:1 contrast on #1a1a1a surface, effectively invisible)

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

frontend/src/routes/DropZone.svelte

@@ -146,7 +146,7 @@ $effect(() => {
 		? 'border-primary bg-accent-bg py-10 text-primary'
 		: windowDragging
 			? 'border-primary/60 bg-accent-bg/50 py-10 text-primary/80'
-			: 'border-ink/20 py-6 text-ink-3 hover:border-primary hover:text-primary'}"
+			: 'border-ink/30 py-6 text-ink-3 hover:border-primary hover:text-primary'}"
 	ondragover={handleDragOver}
 	ondragleave={handleDragLeave}
 	ondrop={handleDrop}

frontend/src/routes/layout.css

@@ -96,8 +96,8 @@
 		--c-overlay: #242424;
 		--c-muted: #252525;
 
-		--c-line: #2e2e2e;
-		--c-line-2: #222222;
+		--c-line: #3d3d3d;
+		--c-line-2: #2e2e2e;
 
 		--c-ink: #f0efe9;
 		--c-ink-2: #9ca3af; /* gray-400 — 7.5:1 on dark surface — WCAG AAA ✓ */
@@ -124,8 +124,8 @@
 	--c-overlay: #242424;
 	--c-muted: #252525;
 
-	--c-line: #2e2e2e;
-	--c-line-2: #222222;
+	--c-line: #3d3d3d;
+	--c-line-2: #2e2e2e;
 
 	--c-ink: #f0efe9;
 	--c-ink-2: #9ca3af;