feat(mobile): add SureTextField primitive and migrate the proxy-headers editor (#2378)

Adds SureTextField, the form-control primitive following SureButton in the
mobile design-system sequence: a TextFormField wrapper that builds a complete,
brightness-aware InputDecoration from the active SureColors palette (filled
bg-container, borderSecondary hairline -> borderPrimary on focus, destructive
error border, textSubdued placeholder, radiusLg corners) with a DS-style label
rendered above the field and associated to it for screen readers (Material
labelText parity via ExcludeSemantics + Semantics(label:)).

Migrates the custom proxy-headers editor off raw TextFormField as proof.
Filter chips and a segmented control will follow as separate PRs.

Part of #2235.

mobile/test/widgets/custom_proxy_headers_editor_test.dart

@@ -0,0 +1,49 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sure_mobile/models/custom_proxy_header.dart';
+import 'package:sure_mobile/theme/sure_theme.dart';
+import 'package:sure_mobile/widgets/custom_proxy_headers_editor.dart';
+import 'package:sure_mobile/widgets/sure_text_field.dart';
+
+// Proof that migrating the editor's fields to SureTextField preserved behavior:
+// the fields still render, onChanged still fires, and validators still run.
+void main() {
+  Future<void> pump(WidgetTester tester, Widget child) {
+    return tester.pumpWidget(
+      MaterialApp(theme: SureTheme.light, home: Scaffold(body: child)),
+    );
+  }
+
+  testWidgets('renders SureTextField rows for each header', (tester) async {
+    await pump(
+      tester,
+      CustomProxyHeadersEditor(
+        initialHeaders: [CustomProxyHeader(name: 'X-Token', value: 'abc')],
+        onChanged: (_) {},
+      ),
+    );
+    // Name + value field per header row.
+    expect(find.byType(SureTextField), findsNWidgets(2));
+    expect(find.text('Header name'), findsOneWidget);
+    expect(find.text('Header value'), findsOneWidget);
+  });
+
+  testWidgets('editing a field fires onChanged with the parsed headers',
+      (tester) async {
+    List<CustomProxyHeader>? latest;
+    await pump(
+      tester,
+      CustomProxyHeadersEditor(
+        initialHeaders: [CustomProxyHeader(name: 'X-Token', value: 'abc')],
+        onChanged: (headers) => latest = headers,
+      ),
+    );
+
+    await tester.enterText(find.byType(TextField).first, 'X-Renamed');
+    await tester.pump();
+
+    expect(latest, isNotNull);
+    expect(latest!.single.name, 'X-Renamed');
+    expect(latest!.single.value, 'abc');
+  });
+}