feat: right-click context menu + CONSTRUCTIONLINE alias

Viewport right-click now shows a proper widget-based context menu:
- When a command is active: Cancel / Enter
- Otherwise: Repeat last commands, Delete/Move/Copy (if selected),
  Select All, Zoom Extents

CONSTRUCTIONLINE added as an alias for XLINE (XL).
Canvas-drawn placeholder removed; context menu closes on Escape,
click-outside, or when any action is taken.

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

ROADMAP.md

@@ -96,7 +96,7 @@ Underlay (PDF/DWF/DGN)
 | SOLID (2D dolu dörtgen) | ✅ |
 | RECTANG (REC) | ✅ |
 | POLYGON (POL) | ✅ |
-| CONSTRUCTION LINE (tam sonsuz) | ⬜ |
+| CONSTRUCTION LINE (tam sonsuz) | ✅ |
 
 ---
 
@@ -276,7 +276,7 @@ Underlay (PDF/DWF/DGN)
 | MATCHPROP (özellik kopyala) | ✅ |
 | BYLAYER hızlı atama | ✅ |
 | Çoklu seçim (window/crossing) | ✅ |
-| Sağ tık bağlam menüsü | ⬜ |
+| Sağ tık bağlam menüsü | ✅ |
 | Araç çubuğu özelleştirme | ⬜ |
 | Tema / Renk şeması seçimi | ⬜ |
 | Klavye kısayol düzenleyici | ⬜ |