feat: add clickable space-mode button to status bar

Replaces the static MODEL/LAYOUT pill with an interactive button that
shows and toggles the current editing context:

  Model tab     → "MODEL"  (non-clickable, informational)
  Layout PSPACE → "PAPER"  (click → MspaceCommand: enter MSPACE)
  Layout MSPACE → "MODEL"  (click → ExitViewport: return to PSPACE)

Highlights in blue when in MSPACE to signal active model editing.
Mirrors the AutoCAD status-bar convention for MODEL/PAPER switching.

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

src/app/view.rs

@@ -313,6 +313,7 @@ impl H7CAD {
                     self.layout_rename_state.as_ref(),
                     tab.scene.first_viewport_scale(),
                     tab.scene.viewport_count(),
+                    tab.scene.active_viewport.is_some(),
                 )
             ]
             .width(Fill)