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 <[email protected]>

src/app/update.rs

@@ -5471,17 +5471,13 @@ impl OpenCADStudio {
 
             // ── About window ──────────────────────────────────────────────
             Message::AboutOpen => {
-                if let Some(id) = self.about_window {
-                    return window::gain_focus(id);
-                }
-                let (id, task) = window::open(window::Settings {
-                    size: iced::Size::new(340.0, 240.0),
-                    resizable: false,
-                    level: window::Level::AlwaysOnTop,
-                    ..Default::default()
-                });
-                self.about_window = Some(id);
-                task.map(|_| Message::Noop)
+                self.about_open = true;
+                Task::none()
+            }
+
+            Message::AboutClose => {
+                self.about_open = false;
+                Task::none()
             }
 
             Message::AboutCopyInfo => {