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/ribbon/mod.rs

@@ -246,7 +246,9 @@ impl Ribbon {
                 .align_y(iced::Center)
                 .spacing(6),
             |row_acc, (i, module)| {
-                if module.id() == "layout" && !is_paper {
+                // The Layout module no longer has a ribbon tab — its paper-space
+                // tools live in the right-edge side toolbar (see ui::side_toolbar).
+                if module.id() == "layout" {
                     return row_acc;
                 }