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/core/wikilink_syntax.dart

@@ -0,0 +1,24 @@
+import 'package:markdown/markdown.dart' as md;
+
+/// Parses `[[Note Title]]` wikilinks into anchor elements with a custom
+/// `wikilink://<encoded-title>` href, handled by the app's onTapLink callback.
+class WikilinkSyntax extends md.InlineSyntax {
+  WikilinkSyntax() : super(r'\[\[([^\[\]\n]+)\]\]');
+
+  @override
+  bool onMatch(md.InlineParser parser, Match match) {
+    final title = match[1]!.trim();
+    if (title.isEmpty) return false;
+    final anchor = md.Element.text('a', title);
+    anchor.attributes['href'] = 'wikilink://${Uri.encodeComponent(title)}';
+    parser.addNode(anchor);
+    return true;
+  }
+}
+
+/// GitHub-flavored markdown extended with [[wikilink]] support.
+/// Use this as the extensionSet wherever note bodies are rendered.
+final wikilinkExtensionSet = md.ExtensionSet(
+  md.ExtensionSet.gitHubFlavored.blockSyntaxes,
+  [WikilinkSyntax(), ...md.ExtensionSet.gitHubFlavored.inlineSyntaxes],
+);