fix(studyset): more room between the title's row and the path's

The studyset's title row (title, Share, options) ran straight into the
row of the path and the parts' switch. They now have 8 dp more between
them: Material 3 sets parts of a header apart by space, and a line
there would compete with the boxes just below.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/src/feature/studyset/widget/studyset_screen.dart

@@ -349,6 +349,9 @@ class _StudysetViewState() extends State<_StudysetView> {
   /// How wide each part's button is in the path's line.
   static const _partWidth = 120.0;
 
+  /// The room between the title's row and the rows below it.
+  static const headerGap = 8.0;
+
   /// From expanded windows on: how wide the column of the description and
   /// the topics is, and the room around and between the two columns.
   static const _sideColumnWidth = 420.0;
@@ -467,6 +470,9 @@ class _StudysetViewState() extends State<_StudysetView> {
         body: Column(
           crossAxisAlignment: CrossAxisAlignment.stretch,
           children: [
+            // Room between the title's row and what follows it, setting
+            // them apart by space rather than a line (decided 2026-10-12).
+            const SizedBox(height: headerGap),
             if (!wide)
               Padding(
                 padding: const EdgeInsets.fromLTRB(12, 0, 12, 4),