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>

pubspec.lock

@@ -353,7 +353,7 @@ packages:
     source: hosted
     version: "1.3.0"
   markdown:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: markdown
       sha256: "935e23e1ff3bc02d390bad4d4be001208ee92cc217cb5b5a6c19bc14aaa318c1"