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/update/file.rs

@@ -40,7 +40,6 @@ impl OpenCADStudio {
             },
             plugin_repos: self.plugin_repos.clone(),
             texteditmode: self.texteditmode,
-            rmb_enter: self.rmb_enter,
             bg_color: self.default_bg_color.map(f4_to_u3),
             paper_bg_color: self.default_paper_bg_color.map(f4_to_u3),
         }
@@ -59,7 +58,6 @@ impl OpenCADStudio {
         self.disabled_plugins = s.disabled_plugins.iter().cloned().collect();
         self.plugin_repos = s.plugin_repos.clone();
         self.texteditmode = s.texteditmode;
-        self.rmb_enter = s.rmb_enter;
         self.default_bg_color = s.bg_color.map(u3_to_f4);
         self.default_paper_bg_color = s.paper_bg_color.map(u3_to_f4);
         // Push the restored background onto every drawing tab that exists now