feat(ui): right-edge side toolbar; drop the paper-space ribbon tab

Switching to a paper layout used to hijack the ribbon with a contextual
"Layout" tab. Replace that with a reusable right-edge vertical toolbar:
a single centred column of icon buttons floated over the canvas, built
from a flat ToolDef list and dispatching through the existing
RibbonToolClick path (ui::side_toolbar). Paper space now shows Viewport /
Page Setup / Export PDF there and leaves the ribbon on the user's tab;
the contextual layout tab and its three auto-switch hooks are removed.
The toolbar is generic infrastructure for future context action sets.

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

src/app/view.rs

@@ -1119,6 +1119,16 @@ impl OpenCADStudio {
             }
         }
 
+        // Paper-space context actions: a right-edge vertical toolbar
+        // (viewport / page setup / plot) instead of a contextual ribbon tab.
+        if is_paper && !tab.is_start {
+            if let Some(tb) = crate::ui::side_toolbar::view(
+                &crate::modules::layout::paper_space_tools(),
+            ) {
+                viewport_stack = viewport_stack.push(tb);
+            }
+        }
+
         // Quick Properties: compact floating property panel on selection,
         // anchored at the canvas top-left so it doesn't track the cursor.
         if self.quick_properties && !tab.is_start {