added a swipeable media gallery for chat and folder images and videos with server previews, on-demand originals and zoom, and marked document previews in chat so they no longer pass for photos

lib/widget/media_gallery/gallery_video_page.dart

@@ -0,0 +1,155 @@
+import 'dart:async';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+
+import '../file_viewer/media_player.dart';
+import 'gallery_cache.dart';
+import 'gallery_item.dart';
+
+/// A video in the gallery: thumbnail and play button; the file is only
+/// downloaded on play. Leaving the page tears the player down.
+class GalleryVideoPage extends StatefulWidget {
+  final GalleryItem item;
+  final bool isActive;
+
+  const GalleryVideoPage({
+    required this.item,
+    required this.isActive,
+    super.key,
+  });
+
+  @override
+  State<GalleryVideoPage> createState() => _GalleryVideoPageState();
+}
+
+class _GalleryVideoPageState extends State<GalleryVideoPage> {
+  String? _path;
+  bool _playing = false;
+  bool _failed = false;
+  ValueListenable<double?>? _progress;
+
+  @override
+  void initState() {
+    super.initState();
+    GalleryOriginals.instance.addListener(_onFlightsChanged);
+    _progress = GalleryOriginals.instance.progressOf(widget.item);
+  }
+
+  @override
+  void didUpdateWidget(covariant GalleryVideoPage oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (!widget.isActive && _playing) _playing = false;
+  }
+
+  @override
+  void dispose() {
+    GalleryOriginals.instance.removeListener(_onFlightsChanged);
+    super.dispose();
+  }
+
+  void _onFlightsChanged() {
+    final progress = GalleryOriginals.instance.progressOf(widget.item);
+    if (progress != _progress && mounted) setState(() => _progress = progress);
+  }
+
+  Future<void> _play() async {
+    setState(() => _failed = false);
+    try {
+      final file =
+          await GalleryOriginals.instance.cached(widget.item) ??
+          await GalleryOriginals.instance.load(widget.item);
+      if (!mounted) return;
+      setState(() {
+        _path = file.path;
+        _playing = widget.isActive;
+      });
+    } on Object {
+      if (mounted) setState(() => _failed = true);
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final path = _path;
+    if (_playing && path != null) {
+      return MediaPlayer(
+        key: ValueKey(path),
+        path: path,
+        isAudio: false,
+        autoPlay: true,
+      );
+    }
+    final progress = _progress;
+    return Stack(
+      fit: StackFit.expand,
+      children: [
+        Image(
+          image: galleryPreviewProvider(context, widget.item),
+          fit: BoxFit.contain,
+          gaplessPlayback: true,
+          errorBuilder: (_, _, _) =>
+              const Center(child: Icon(Icons.movie_outlined, size: 96)),
+        ),
+        Center(
+          child: progress != null
+              ? ValueListenableBuilder<double?>(
+                  valueListenable: progress,
+                  builder: (context, value, _) => _PlayCircle(
+                    child: CircularProgressIndicator(
+                      value: value,
+                      color: Colors.white,
+                      strokeWidth: 3,
+                      semanticsLabel: 'Video wird geladen',
+                    ),
+                  ),
+                )
+              : _PlayCircle(
+                  onTap: () => unawaited(_play()),
+                  child: const Icon(
+                    Icons.play_arrow,
+                    color: Colors.white,
+                    size: 40,
+                    semanticLabel: 'Abspielen',
+                  ),
+                ),
+        ),
+        if (_failed)
+          Positioned(
+            left: 24,
+            right: 24,
+            bottom: 32,
+            child: SafeArea(
+              child: Text(
+                'Video konnte nicht geladen werden. Tippe erneut auf Abspielen.',
+                textAlign: TextAlign.center,
+                style: Theme.of(context).textTheme.bodyMedium,
+              ),
+            ),
+          ),
+      ],
+    );
+  }
+}
+
+class _PlayCircle extends StatelessWidget {
+  final Widget child;
+  final VoidCallback? onTap;
+
+  const _PlayCircle({required this.child, this.onTap});
+
+  @override
+  Widget build(BuildContext context) => Material(
+    color: Colors.black54,
+    shape: const CircleBorder(),
+    clipBehavior: Clip.antiAlias,
+    child: InkWell(
+      onTap: onTap,
+      child: SizedBox(
+        width: 72,
+        height: 72,
+        child: Center(child: SizedBox(width: 40, height: 40, child: child)),
+      ),
+    ),
+  );
+}