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/env-var-editor.tsx

@@ -0,0 +1,76 @@
+"use client";
+
+import { Input } from "@/components/ui/input";
+import { Switch } from "@/components/ui/switch";
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from "@/components/ui/select";
+import type { McpEnvVar } from "@/lib/api/settings";
+
+type Props = {
+  spec: McpEnvVar;
+  value: string;
+  onChange: (v: string) => void;
+  disabled?: boolean;
+};
+
+export function EnvVarEditor({ spec, value, onChange, disabled }: Props) {
+  if (spec.type === "bool") {
+    const checked = value === "true";
+    return (
+      <Switch
+        checked={checked}
+        onCheckedChange={(c) => onChange(c ? "true" : "false")}
+        disabled={disabled}
+      />
+    );
+  }
+
+  if (spec.enum_values && spec.enum_values.length > 0) {
+    return (
+      <Select value={value} onValueChange={onChange} disabled={disabled}>
+        <SelectTrigger className="w-[220px]">
+          <SelectValue />
+        </SelectTrigger>
+        <SelectContent>
+          {spec.enum_values.map((v) => (
+            <SelectItem key={v} value={v}>
+              {v}
+            </SelectItem>
+          ))}
+        </SelectContent>
+      </Select>
+    );
+  }
+
+  if (spec.type === "int" || spec.type === "float") {
+    return (
+      <Input
+        type="number"
+        value={value}
+        onChange={(e) => onChange(e.target.value)}
+        min={spec.min ?? undefined}
+        max={spec.max ?? undefined}
+        step={spec.type === "float" ? "0.01" : "1"}
+        disabled={disabled}
+        className="w-[160px] text-start"
+        dir="ltr"
+      />
+    );
+  }
+
+  return (
+    <Input
+      type="text"
+      value={value}
+      onChange={(e) => onChange(e.target.value)}
+      disabled={disabled}
+      className="w-[260px] text-start"
+      dir="ltr"
+    />
+  );
+}