feat(viewport): RMB 1st=Enter, 2nd=menu (no setting)

Replaces the opt-in `rmb_enter` setting (and `RMBENTER` command) with
always-on behavior. While a command is active, the first viewport
right-click acts as Enter (commit / close) and a second consecutive
right-click opens the context menu instead. Idle right-click still opens
the menu; right-drag still orbits.

The cycle is tracked per tab (`right_click_entered`) and restarts on any
other interaction — a left-click pick, a new command, an orbit, or
leaving the viewport — so the next right-click is Enter again.

Refines #185.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/commands/display.rs

@@ -17,27 +17,6 @@ impl OpenCADStudio {
                     .push_output("PAN: drag with the left mouse button. Press Esc to exit.");
             }
 
-            // RMBENTER [ON|OFF] — toggle whether a viewport right-click acts as
-            // Enter (commit / close) while a command is active. Idle right-click
-            // still opens the context menu and right-drag still orbits. The
-            // preference is persisted across runs.
-            cmd if cmd == "RMBENTER" || cmd.starts_with("RMBENTER ") => {
-                let arg = cmd
-                    .split_once(' ')
-                    .map(|(_, r)| r.trim().to_uppercase())
-                    .unwrap_or_default();
-                self.rmb_enter = match arg.as_str() {
-                    "ON" | "1" => true,
-                    "OFF" | "0" => false,
-                    _ => !self.rmb_enter,
-                };
-                self.command_line.push_output(if self.rmb_enter {
-                    "RMBENTER on: right-click acts as Enter while a command is active."
-                } else {
-                    "RMBENTER off: right-click opens the context menu."
-                });
-            }
-
             // ── TABLE cell editing ─────────────────────────────────────────────
             // TABLE CELL <row> <col> <text> — set text for a cell in the selected Table
             cmd if cmd.starts_with("TABLE ") => {