fix(welcome): defer doc-load to didChangeDependencies

_DocReaderSheetState.initState() called _load(), which
synchronously calls Localizations.localeOf(context) before any
await. That triggers dependOnInheritedWidgetOfExactType<
_LocalizationsScope>() in initState(), which Flutter forbids
because InheritedWidget wiring is not finished yet.

Symptom (clicking a doc card in Welcome):
  dependOnInheritedWidgetOfExactType<_LocalizationsScope>()
  or dependOnInheritedElement() was called before
  _DocReaderSheetState.initState() completed.

Fix: defer _load() to didChangeDependencies(), which runs
after initState() and after InheritedWidget hookup. Gate on
_content == null so it runs exactly once per widget lifetime
(didChangeDependencies can also fire on later dependency
changes; we only want the first load).

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/pages/welcome.dart

@@ -935,12 +935,16 @@ class _DocReaderSheet extends StatefulWidget {
 }
 
 class _DocReaderSheetState extends State<_DocReaderSheet> {
-  late final Future<String> _content;
+  Future<String>? _content;
 
   @override
-  void initState() {
-    super.initState();
-    _content = _load();
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    // `_load()` needs `Localizations.localeOf(context)`. That call
+    // touches InheritedWidgets which aren't safe to read in
+    // initState() — must wait until didChangeDependencies. We
+    // gate on `_content == null` so it runs exactly once.
+    _content ??= _load();
   }
 
   Future<String> _load() async {