feat(ui): migrate the last dialogs to modals — fully single-window (Plan B)

Move the unsaved-changes prompt, the Save As browser and the
file-association prompt onto `active_modal` / `modal_content`, reusing
their existing open/close helpers (now just set/clear active_modal). The
daemon `view` no longer dispatches per window — every dialog is an
in-canvas modal, so the app is single-window on native and web alike.

`close_active_modal` handles their ✕: the unsaved prompt cancels the
pending close, the association prompt records that it was answered.

Native + web build; 75 tests pass.

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

src/app/mod.rs

@@ -606,6 +606,9 @@ pub enum ModalKind {
     MlStyle,
     MLeaderStyle,
     DimStyle,
+    Unsaved,
+    SaveDialog,
+    AssocPrompt,
 }
 
 /// Identifies a DimStyle field that can be edited in the dialog.
@@ -1687,18 +1690,10 @@ impl OpenCADStudio {
         // One-time prompt offering to make Open CAD Studio the default app for
         // .dwg / .dxf. Shown only on the first launch that hasn't answered it
         // yet; the flag is persisted so we never ask twice.
-        let assoc_prompt: Task<Message> = if s.default_assoc_prompted {
-            Task::none()
-        } else {
-            let (id, open) = window::open(window::Settings {
-                size: iced::Size::new(440.0, 210.0),
-                resizable: false,
-                level: window::Level::AlwaysOnTop,
-                ..Default::default()
-            });
-            s.assoc_prompt_window = Some(id);
-            open.map(|_| Message::Noop)
-        };
+        let assoc_prompt: Task<Message> = Task::none();
+        if !s.default_assoc_prompted {
+            s.active_modal = Some(ModalKind::AssocPrompt);
+        }
         (
             s,
             Task::batch([open_main, check_update, focus_cmd, cli_open, assoc_prompt]),