Selection tools on the right-edge toolbar instead of contextual tabs

The PDF Underlay and External Reference ribbon tabs are gone. While only
PDF underlays are selected their tools (Display in Monochrome, Create
Clipping Boundary, Remove Clipping, Show Underlay, Enable Snap, External
References, Edit Layers, Import As Objects) sit on the right-edge toolbar,
the switches drawn as on while on; while only xrefs are selected the xref
tools do. Contrast and fade stay in Properties. Toolbar tooltips are
translated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/mod.rs

@@ -3827,11 +3827,8 @@ pub enum Message {
     PdfAttachPick,
     /// PDFIMPORT File: pick the PDF to import.
     PdfImportPick,
-    /// The contextual PDF Underlay tab.
-    RibbonSelectUnderlayTab,
     /// An edit in one of the PDF dialogs.
     PdfDialog(crate::ui::window::pdf_dialogs::PdfDialogMsg),
-    UnderlayTab(crate::ui::ribbon::UnderlayTabMsg),
     PdfImportPickResult(Result<(std::path::PathBuf, std::sync::Arc<Vec<u8>>), String>),
     /// Result of the PDFATTACH file picker.
     PdfAttachPickResult(Result<(std::path::PathBuf, std::sync::Arc<Vec<u8>>), String>),