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>

src/ui/command_line.rs

@@ -17,7 +17,7 @@ pub struct CommandLine {
     pub input: String,
     pub history: Vec<HistoryEntry>,
     /// Commands the user has typed (for ↑/↓ recall).
-    cmd_recall: Vec<String>,
+    pub cmd_recall: Vec<String>,
     /// Current position in `cmd_recall` while navigating (None = not navigating).
     recall_cursor: Option<usize>,
     /// Saved draft input before the user started navigating history.