feat(settings): implement Environment tab with edit + drift detection

Add drift-badge, env-var-editor, env-var-row components and replace the
environment-tab stub; install shadcn Switch which was missing.

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

web-ui/src/app/settings/_components/drift-badge.tsx

@@ -0,0 +1,21 @@
+"use client";
+
+import { AlertTriangle, CheckCircle2 } from "lucide-react";
+import { Badge } from "@/components/ui/badge";
+
+export function DriftBadge({ drift }: { drift: boolean }) {
+  if (drift) {
+    return (
+      <Badge variant="outline" className="text-warn border-warn/40 gap-1">
+        <AlertTriangle className="w-3 h-3" />
+        Drift
+      </Badge>
+    );
+  }
+  return (
+    <Badge variant="outline" className="text-success border-success/40 gap-1">
+      <CheckCircle2 className="w-3 h-3" />
+      Synced
+    </Badge>
+  );
+}