fix(client): 出入库录单 golden 随日期漂移——表单支持钉死单据日期

根因：表单 _orderDate 默认 DateTime.now()，golden 07-04 录制后每天必挂
（整屏 diff 只有日期格一个数字）。表单加 initialDate 测试注入口，
golden 测试钉死 2026-07-04，基准图零重录、永不随日期漂移。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJ1g8XV1YhhmHRzhwWEW7o

client/test/golden/stock_in_form_golden_test.dart

@@ -73,7 +73,7 @@ void main() {
   goldenAcrossThemes(
     'stock-in form 录单(桌面)',
     goldenPrefix: 'stock_in_form',
-    child: () => const Scaffold(body: StockInFormScreen()),
+    child: () => Scaffold(body: StockInFormScreen(initialDate: DateTime(2026, 7, 4))),
     overrides: _overrides,
     logical: const Size(1280, 920),
   );
@@ -81,7 +81,7 @@ void main() {
   goldenAcrossThemes(
     'stock-in form 录单(移动)',
     goldenPrefix: 'stock_in_form_mobile',
-    child: () => const Scaffold(body: StockInFormScreen()),
+    child: () => Scaffold(body: StockInFormScreen(initialDate: DateTime(2026, 7, 4))),
     overrides: _overrides,
     logical: const Size(390, 1400),
   );

client/test/golden/stock_out_form_golden_test.dart

@@ -53,7 +53,7 @@ void main() {
   goldenAcrossThemes(
     'stock-out form 录单(桌面)',
     goldenPrefix: 'stock_out_form',
-    child: () => const Scaffold(body: StockOutFormScreen()),
+    child: () => Scaffold(body: StockOutFormScreen(initialDate: DateTime(2026, 7, 4))),
     overrides: _overrides,
     logical: const Size(1280, 920),
   );
@@ -61,7 +61,7 @@ void main() {
   goldenAcrossThemes(
     'stock-out form 录单(移动)',
     goldenPrefix: 'stock_out_form_mobile',
-    child: () => const Scaffold(body: StockOutFormScreen()),
+    child: () => Scaffold(body: StockOutFormScreen(initialDate: DateTime(2026, 7, 4))),
     overrides: _overrides,
     logical: const Size(390, 1400),
   );