fix(css): set form control bg/color to surface tokens in base layer

Browser-default form controls (input, textarea, select) render with a white
background that ignores CSS custom properties in dark mode. Adding bg-surface
and text-ink to the base layer ensures they theme correctly without touching
every component.

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

frontend/src/routes/layout.css

@@ -183,4 +183,12 @@
 		font-family: var(--font-sans);
 		font-weight: 600;
 	}
+
+	/* Form controls — always use surface bg and ink text so they theme correctly */
+	input,
+	textarea,
+	select {
+		background-color: var(--c-surface);
+		color: var(--c-ink);
+	}
 }