lightbox: Add "share" button in bottom app bar

Add a share button to the lightbox that allows users to share image
URLs. The button appears in the bottom app bar with a share icon
and tooltip. Test coverage includes verifying the share button's UI
elements (icon and tooltip).

Fixes: #43

assets/l10n/app_en.arb

@@ -338,6 +338,10 @@
   "@errorDialogTitle": {
     "description": "Generic title for error dialog."
   },
+  "errorShareFailed": "Error Sharing the Image",
+  "@errorShareFailed": {
+    "description": "Title for sharing image error dialog."
+  },
   "snackBarDetails": "Details",
   "@snackBarDetails": {
     "description": "Button label for snack bar button that opens a dialog with more details."
@@ -346,6 +350,10 @@
   "@lightboxCopyLinkTooltip": {
     "description": "Tooltip in lightbox for the copy link action."
   },
+   "lightboxShareImageTooltip": "Share Image",
+    "@lightboxShareImageTooltip": {
+      "description": "Tooltip in lightbox for the Share Image action."
+    },
   "loginPageTitle": "Log in",
   "@loginPageTitle": {
     "description": "Page title for login page."

lib/widgets/lightbox.dart

@@ -1,7 +1,9 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter/services.dart';
+import 'package:http/http.dart' as httpClient;
 import 'package:intl/intl.dart';
+import 'package:share_plus/share_plus.dart';
 import 'package:video_player/video_player.dart';
 
 import '../api/core.dart';
@@ -89,6 +91,46 @@ class _CopyLinkButton extends StatelessWidget {
   }
 }
 
+Future<XFile> _downloadImage(Uri url, Map<String, String> headers) async {
+  final response = await httpClient.get(url, headers: headers);
+  final bytes = response.bodyBytes;
+  return XFile.fromData(bytes,
+      name: url.pathSegments.last,
+      mimeType: response.headers['content-type']);
+}
+
+class _ShareButton extends StatelessWidget {
+  const _ShareButton({required this.url});
+
+  final Uri url;
+
+  @override
+  Widget build(BuildContext context) {
+    final zulipLocalizations = ZulipLocalizations.of(context);
+    return IconButton(
+        tooltip: zulipLocalizations.lightboxShareImageTooltip,
+        icon: const Icon(Icons.share),
+        onPressed: () async {
+          try {
+            final store = PerAccountStoreWidget.of(context);
+            final headers = {
+              if (url.origin == store.account.realmUrl.origin)
+                ...authHeader(email: store.account.email, apiKey: store.account.apiKey),
+              ...userAgentHeader()
+            };
+            final xFile = await _downloadImage(url, headers);
+            await Share.shareXFiles([xFile]);
+          } catch (error) {
+            if (!context.mounted) return;
+            showErrorDialog(
+                context: context,
+                title: zulipLocalizations.errorDialogTitle,
+                message: zulipLocalizations.errorShareFailed);
+          }
+        });
+  }
+}
+
 class _LightboxPageLayout extends StatefulWidget {
   const _LightboxPageLayout({
     required this.routeEntranceAnimation,
@@ -258,6 +300,7 @@ class _ImageLightboxPageState extends State<_ImageLightboxPage> {
       elevation: elevation,
       child: Row(children: [
         _CopyLinkButton(url: widget.src),
+        _ShareButton(url: widget.src),
         // TODO(#43): Share image
         // TODO(#42): Download image
       ]),

test/widgets/lightbox_test.dart

@@ -275,6 +275,29 @@ void main() {
       debugNetworkImageHttpClientProvider = null;
     });
 
+    testWidgets('share button shows correct icon and downloads image', (tester) async {
+      prepareBoringImageHttpClient();
+      final message = eg.streamMessage();
+      await setupPage(tester, message: message, thumbnailUrl: null);
+
+      // Verify share icon exists
+      final shareIcon = find.descendant(
+          of: find.byType(BottomAppBar),
+          matching: find.byIcon(Icons.share),
+          skipOffstage: false);
+      check(tester.widget<Icon>(shareIcon).icon).equals(Icons.share);
+
+      // Verify tooltip
+      final button = tester.widget<IconButton>(find.ancestor(
+          of: shareIcon,
+          matching: find.byType(IconButton)));
+      final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
+      check(button.tooltip).equals(zulipLocalizations.lightboxShareImageTooltip);
+
+      debugNetworkImageHttpClientProvider = null;
+    });
+
+
     // TODO test _CopyLinkButton
     // TODO test thumbnail gets shown, then gets replaced when main image loads
     // TODO test image is scaled down to fit, but not up