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/mod.rs

@@ -294,6 +294,8 @@ pub(super) struct OpenCADStudio {
     color_pick_target: Option<ColorPickTarget>,
     shortcuts_window: Option<window::Id>,
     about_window: Option<window::Id>,
+    /// In-canvas About modal (Plan B: shared overlay instead of an OS window).
+    about_open: bool,
     plugin_manager_window: Option<window::Id>,
     /// New-release notification window — opened on startup when the
     /// GitHub releases API reports a newer version than this build.
@@ -992,6 +994,7 @@ pub enum Message {
     ShortcutsPanelClose,
     // ── About window ────────────────────────────────────────────────────
     AboutOpen,
+    AboutClose,
     AboutCopyInfo,
     // ── Plugin Manager window ───────────────────────────────────────────
     PluginManagerOpen,
@@ -1413,6 +1416,7 @@ impl OpenCADStudio {
             color_pick_target: None,
             shortcuts_window: None,
             about_window: None,
+            about_open: false,
             plugin_manager_window: None,
             update_notice_window: None,
             assoc_prompt_window: None,

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 => {

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> {