feat(ui): shared in-canvas modal overlay; migrate About (Plan B)

Start Plan B — replace the daemon's pop-up OS windows with in-canvas
modal overlays so dialogs work identically on native (one main window)
and web (canvas only).

- New `ui::modal::modal(base, content, on_close)`: dims the view behind
  a backdrop that blocks clicks but does NOT dismiss, frames the content,
  and adds a top-right ✕ as the sole close affordance. Sizes to the
  content (no `Fill`), so dialogs shrink to fit.
- view_main stacks the active modal on top, so both run paths show it.
- Migrate About as the pilot: `about_open` bool + AboutOpen/AboutClose
  instead of an OS window; About's view shrinks to content.

Remaining dialogs migrate to the same pattern next. Native + 75 tests OK.

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

src/app/view.rs

@@ -1499,7 +1499,7 @@ impl OpenCADStudio {
             iced::widget::Space::new().width(0).height(0).into()
         };
 
-        stack![
+        let composed = stack![
             main_ui,
             self.app_menu.view(),
             snap_layer,
@@ -1512,8 +1512,20 @@ impl OpenCADStudio {
             layout_ctx_layer,
             qselect_layer,
             open_progress_layer,
-        ]
-        .into()
+        ];
+
+        // ── In-canvas modal dialogs (Plan B) ───────────────────────────────
+        // Former pop-up windows render as overlays here, so they work on both
+        // the native (single main window) and web builds.
+        if self.about_open {
+            crate::ui::modal::modal(
+                composed,
+                crate::ui::about::view_window(),
+                Message::AboutClose,
+            )
+        } else {
+            composed.into()
+        }
     }
 
     pub fn subscription(&self) -> Subscription<Message> {