feat(app): 完善播放协议搜索与空状态

lib/pages/discover/search_page.dart

@@ -20,6 +20,11 @@ import 'discover_binding.dart';
 import 'discover_logic.dart';
 import 'widgets/search_result_card.dart';
 
+const double _clearHit = 20;
+const double _clearIconSize = 16;
+const double _clearEndGap = 10;
+const double _loadingInset = 2;
+
 class SearchPage extends StatefulWidget {
   const SearchPage({super.key});
 
@@ -119,11 +124,10 @@ class _SearchPageState extends State<SearchPage> {
   }
 
   Widget _field(AppLocalizations l10n) {
-    final hasText = _logic.search.text.isNotEmpty;
     return Container(
       key: const Key('search-field'),
       height: AppDimens.searchHeight,
-      padding: const EdgeInsets.symmetric(horizontal: AppDimens.spaceMd),
+      padding: const EdgeInsets.only(left: AppDimens.spaceMd),
       decoration: BoxDecoration(
         color: AppColors.contentSurface,
         borderRadius: BorderRadius.circular(AppDimens.searchRadius),
@@ -143,7 +147,9 @@ class _SearchPageState extends State<SearchPage> {
               focusNode: _logic.searchFocus,
               autofocus: true,
               onChanged: (_) => _logic.onSearchChanged(),
+              onTapOutside: (_) => _logic.searchFocus.unfocus(),
               textInputAction: TextInputAction.search,
+              textAlignVertical: TextAlignVertical.center,
               style: const TextStyle(
                 color: AppColors.textPrimary,
                 fontSize: 14,
@@ -163,35 +169,64 @@ class _SearchPageState extends State<SearchPage> {
               scrollPadding: EdgeInsets.zero,
             ),
           ),
-          if (_logic.searchLoading)
-            const SizedBox.square(
-              dimension: 16,
-              child: CircularProgressIndicator(
-                strokeWidth: 2,
-                color: AppColors.textPrimary,
-              ),
-            )
-          else if (hasText)
-            IconButton(
-              key: const Key('search-clear'),
-              padding: EdgeInsets.zero,
-              constraints: const BoxConstraints.tightFor(width: 24, height: 24),
-              onPressed: () {
-                _logic.search.clear();
-                _logic.onSearchChanged();
-                _logic.searchFocus.requestFocus();
-              },
-              icon: const Icon(
-                Icons.close_rounded,
-                color: AppColors.textSecondary,
-                size: 16,
-              ),
-            ),
+          _fieldTrailing(),
         ],
       ),
     );
   }
 
+  /// 有内容时把清除按钮放在输入框内部右侧，并留出一点边距。
+  /// 刷新时不拿转圈替换叉号，避免宽度变化把按钮抖开。
+  Widget _fieldTrailing() {
+    final hasText = _logic.search.text.isNotEmpty;
+    if (!hasText && !_logic.searchLoading) {
+      return const SizedBox(width: AppDimens.spaceMd);
+    }
+    return SizedBox(
+      width: _clearHit + _clearEndGap,
+      child: Align(
+        alignment: Alignment.centerRight,
+        child: Padding(
+          padding: const EdgeInsets.only(right: _clearEndGap),
+          child: hasText ? _clearButton() : _loadingMark(),
+        ),
+      ),
+    );
+  }
+
+  Widget _clearButton() {
+    return GestureDetector(
+      key: const Key('search-clear'),
+      behavior: HitTestBehavior.opaque,
+      onTap: () {
+        _logic.search.clear();
+        _logic.onSearchChanged();
+        _logic.searchFocus.requestFocus();
+      },
+      child: const SizedBox.square(
+        dimension: _clearHit,
+        child: Icon(
+          Icons.close_rounded,
+          color: AppColors.textSecondary,
+          size: _clearIconSize,
+        ),
+      ),
+    );
+  }
+
+  Widget _loadingMark() {
+    return const SizedBox.square(
+      dimension: _clearHit,
+      child: Padding(
+        padding: EdgeInsets.all(_loadingInset),
+        child: CircularProgressIndicator(
+          strokeWidth: 2,
+          color: AppColors.textPrimary,
+        ),
+      ),
+    );
+  }
+
   Widget _body(AppLocalizations l10n) {
     if (!_logic.hasSearchQuery) {
       return _stateScroll(