fix(#433): address Gemini review (BuildContext async-gap + web download)

- LogExport.shareLogs/shareFile now resolve ScaffoldMessenger and l10n strings
  off the context BEFORE the flush await, and _exportFile takes them as values
  (no BuildContext use across the async gap; avoids a deactivated-ancestor crash
  on Back-during-flush).
- Web download: append the anchor to the document before click() (Firefox needs
  a connected anchor) and revoke the object URL on a delay (synchronous revoke
  can abort the download in Safari/iOS).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/utils/log_export.dart

@@ -57,13 +57,18 @@ class LogExport {
       );
       return;
     }
+    // Resolve everything off `context` BEFORE the async gap so nothing touches a
+    // possibly-deactivated widget after the await.
+    final messenger = ScaffoldMessenger.of(context);
+    final l10n = context.l10n;
     final file = await FileLogService.instance.flushAndGetActiveFile();
-    if (!context.mounted) return;
     await _exportFile(
-      context,
+      messenger,
       file: file,
       fileName: _defaultFileName,
-      subject: context.l10n.debugLog_shareSubject,
+      subject: l10n.debugLog_shareSubject,
+      unavailableMessage: l10n.debugLog_logUnavailable,
+      savedMessage: l10n.debugLog_logSaved,
     );
   }
 
@@ -75,25 +80,28 @@ class LogExport {
     String? subject,
     String? fileName,
   }) async {
+    final messenger = ScaffoldMessenger.of(context);
+    final l10n = context.l10n;
     await _exportFile(
-      context,
+      messenger,
       file: file,
       fileName: fileName ?? file.uri.pathSegments.last,
-      subject: subject ?? context.l10n.debugLog_shareSubject,
+      subject: subject ?? l10n.debugLog_shareSubject,
+      unavailableMessage: l10n.debugLog_logUnavailable,
+      savedMessage: l10n.debugLog_logSaved,
     );
   }
 
   static Future<void> _exportFile(
-    BuildContext context, {
+    ScaffoldMessengerState messenger, {
     required File? file,
     required String fileName,
     required String subject,
+    required String unavailableMessage,
+    required String savedMessage,
   }) async {
-    final messenger = ScaffoldMessenger.of(context);
-    final unavailable = context.l10n.debugLog_logUnavailable;
-    final savedMessage = context.l10n.debugLog_logSaved;
     if (file == null) {
-      messenger.showSnackBar(SnackBar(content: Text(unavailable)));
+      messenger.showSnackBar(SnackBar(content: Text(unavailableMessage)));
       return;
     }
     if (_isMobile) {

lib/utils/log_web_download_web.dart

@@ -1,9 +1,14 @@
+import 'dart:async';
 import 'dart:js_interop';
 
 import 'package:web/web.dart' as web;
 
 /// Trigger a browser download of [content] (UTF-8 text) as [fileName] by
 /// creating an in-memory Blob and clicking a transient anchor. #433
+///
+/// The anchor is appended to the document before clicking (Firefox ignores
+/// `click()` on a disconnected anchor) and the object URL is revoked on a delay
+/// (revoking synchronously can abort the download in Safari/iOS).
 void downloadTextFile(String fileName, String content) {
   final blob = web.Blob(
     <JSAny>[content.toJS].toJS,
@@ -13,6 +18,13 @@ void downloadTextFile(String fileName, String content) {
   final anchor = web.document.createElement('a') as web.HTMLAnchorElement
     ..href = url
     ..download = fileName;
+  web.document.body?.appendChild(anchor);
   anchor.click();
-  web.URL.revokeObjectURL(url);
+  anchor.remove();
+  unawaited(
+    Future<void>.delayed(
+      const Duration(seconds: 10),
+      () => web.URL.revokeObjectURL(url),
+    ),
+  );
 }