selection and hover use full-row bg markers instead of indicators

- replace IND_SELECTED/IND_HOVER indicators with M_HOVER(6) M_SELECTED(7) bg markers
- disable caret line highlight and native selection rendering
- re-enable sel rendering only during inline edit
- add drag-select support via nodeClicked shift signals
- use lineRangeNoEol helper to exclude EOL from indicator ranges
- add drag tracking state to editor

src/editor.h

@@ -64,6 +64,10 @@ private:
     uint64_t m_hoveredNodeId = 0;
     QSet<uint64_t> m_currentSelIds;
 
+    // ── Drag selection ──
+    bool m_dragging = false;
+    int  m_dragLastLine = -1;
+
     // ── Inline edit state ──
     struct InlineEditState {
         bool       active    = false;