Save prompt on quit: "Save changes to <name>?" for every unsaved design

The title bar's ×, File ▸ Quit, the OS closing the window (Alt+F4, Cmd+Q, the window manager:
close_requested is cancelled until the designs are answered) and app.quit / ui.window close all
go through documents::request_quit, which asks Save / Don't Save / Cancel once per design with
unsaved changes. Save saves (Save As for a never-saved design) and quits only if it saved; Don't
Save discards (no recovery copy is kept); Cancel or Esc stays open. Closing a tab asks the same
question (now titled "Save changes to <name>?"); New and Open never replace an unsaved design
(they open a tab). The web app asks through beforeunload while a design is unsaved. Control:
ui.documents {action: quit, answer: save|dont_save|cancel}, app.quit {force}. Tests: quit with
Save then Don't Save across two tabs, Cancel, a Save that saves nothing, Save As; scenario
s43_quit_asks_to_save.

Fixes #33

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/titlebar.rs

@@ -57,8 +57,11 @@ pub fn window_action(app: &mut SolveApp, ctx: &egui::Context, action: &str) -> R
         "restore" => ctx.send_viewport_cmd(ViewportCommand::Maximized(false)),
         "toggle" => toggle_maximize(ctx),
         "close" => {
-            app.quit_requested = true;
-            ctx.send_viewport_cmd(ViewportCommand::Close);
+            // Unsaved designs are asked about first; the window closes once they are answered.
+            crate::documents::request_quit(app);
+            if app.quit_requested {
+                ctx.send_viewport_cmd(ViewportCommand::Close);
+            }
         }
         other => return Err(format!("unknown window action `{other}` (minimize, maximize, restore, toggle, close)")),
     }