fix: file picker now shows subdirectories on Android

FileType.custom with allowedExtensions causes Android system picker
to hide subdirectories on some devices. Changed to FileType.any with
post-selection extension validation instead.

- Unsupported file types are skipped with a SnackBar hint
- Allowed: jpg, jpeg, png, gif, webp, pdf

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

it0_app/lib/features/chat/presentation/pages/chat_page.dart

@@ -209,18 +209,26 @@ class _ChatPageState extends ConsumerState<ChatPage> {
       return;
     }
 
+    // Use FileType.any so the system picker shows subdirectories for navigation.
+    // We validate file extensions after selection.
     final result = await FilePicker.platform.pickFiles(
-      type: FileType.custom,
+      type: FileType.any,
-      allowedExtensions: ['jpg', 'jpeg', 'png', 'gif', 'webp', 'pdf'],
       allowMultiple: true,
     );
     if (result == null || result.files.isEmpty) return;
 
+    const allowedExts = {'jpg', 'jpeg', 'png', 'gif', 'webp', 'pdf'};
+    int skipped = 0;
+
     final toAdd = result.files.take(remaining);
     for (final file in toAdd) {
       if (file.path == null) continue;
-      final bytes = await File(file.path!).readAsBytes();
       final ext = (file.extension ?? '').toLowerCase();
+      if (!allowedExts.contains(ext)) {
+        skipped++;
+        continue;
+      }
+      final bytes = await File(file.path!).readAsBytes();
       final String mediaType;
       if (ext == 'pdf') {
         mediaType = 'application/pdf';
@@ -241,7 +249,11 @@ class _ChatPageState extends ConsumerState<ChatPage> {
     }
     setState(() {});
 
-    if (result.files.length > remaining && mounted) {
+    if (skipped > 0 && mounted) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(content: Text('仅支持图片(jpg/png/gif/webp)和PDF文件')),
+      );
+    } else if (result.files.length > remaining && mounted) {
       ScaffoldMessenger.of(context).showSnackBar(
         SnackBar(content: Text('已选择 $remaining 个，最多 $_maxAttachments 个')),
       );