feat(ui): migrate 4 more dialogs to in-canvas modals (Plan B)

Consolidate all migrated dialogs into a single `modal_content` builder
matched on `active_modal`, rendered by view_main. Move Layers, Page
Setup, Layout Manager and Plot Style off OS windows onto it (their
construction relocated from the daemon `view` dispatch, which now only
handles the not-yet-migrated dialogs). Each keeps a bounded size.

Open/close handlers set `active_modal` instead of opening/closing
windows; ToggleLayers toggles it.

Native + web build; 75 tests pass.

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

src/app/mod.rs

@@ -597,6 +597,10 @@ pub enum ModalKind {
     Shortcuts,
     PluginManager,
     UpdateNotice,
+    Layers,
+    PageSetup,
+    LayoutManager,
+    Plotstyle,
 }
 
 /// Identifies a DimStyle field that can be edited in the dialog.