Render and navigate [[wikilink]] syntax in notes

- Add WikilinkSyntax (InlineSyntax) that parses [[Note Title]] into
  anchor elements with a wikilink:// href
- wikilinkExtensionSet extends GFM with this syntax; shared by detail
  view and edit preview so rendering is consistent
- NoteDetailScreen handles onTapLink: wikilink:// hrefs look up the
  target note by title (case-insensitive) and push its detail route;
  unresolved links show a "Note not found" snackbar
- Add markdown as an explicit dependency (was previously transitive)

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

lib/screens/notes/note_edit_screen.dart

@@ -4,6 +4,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
 import '../../core/exceptions.dart';
+import '../../core/wikilink_syntax.dart';
 import '../../providers/api_client_provider.dart';
 import '../../providers/notes_provider.dart';
 
@@ -149,7 +150,10 @@ class _NoteEditScreenState extends ConsumerState<NoteEditScreen> {
                     const Divider(),
                     Expanded(
                       child: _preview
-                          ? Markdown(data: _contentController.text)
+                          ? Markdown(
+                              data: _contentController.text,
+                              extensionSet: wikilinkExtensionSet,
+                            )
                           : Padding(
                               padding:
                                   const EdgeInsets.symmetric(horizontal: 16),