fix: define .input component class in CSS (dark-themed inputs/selects)

Missing class caused native selects to render with OS default white background.
Added .input to @layer components with dark bg, border, color-scheme:dark.

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

frontend/src/index.css

@@ -42,6 +42,11 @@
   .positive { @apply text-emerald-400; }
   .negative { @apply text-red-400; }
   .neutral { @apply text-slate-400; }
+  .input {
+    @apply bg-dark-700 border border-slate-700/50 rounded text-slate-200 text-sm
+           px-3 py-1.5 outline-none focus:border-blue-500/60 transition-colors;
+    color-scheme: dark;
+  }
   .section-title {
     @apply text-xs font-semibold text-slate-500 uppercase tracking-widest mb-3;
   }