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/ui/mod.rs

@@ -20,6 +20,7 @@ pub mod properties;
 pub mod ribbon;
 pub mod scale_popup;
 pub mod selection_filter_popup;
+pub mod side_toolbar;
 pub mod shortcuts;
 pub mod snap_popup;
 pub mod statusbar;