feat: in-app update notice — checks GitHub releases on startup

Adds a one-shot update check that runs on app boot and pops up a small
"Update Available" window when the GitHub releases API reports a newer
tag than this build's `CARGO_PKG_VERSION`.

- `src/update_check.rs` — async wrapper around a blocking `ureq` GET to
  `api.github.com/repos/HakanSeven12/H7CAD/releases/latest`, returns
  `Some(version)` when a newer release exists, `None` otherwise (silent
  on network / parse failure so a missing connection never blocks
  launch).
- `H7CAD::boot` chains a `Task::perform` next to the main-window open
  so the check runs in parallel with first-paint.
- New window plumbed like the other floating panels (page setup, about,
  etc.): `update_notice_window`/`update_notice_version` state, title
  entry, `OsWindowClosed` cleanup, and a `ui::update_notice::view_window`
  modal that shows installed vs. latest version plus "Later" /
  "Open Release Page" buttons. Open routes through `open::that` to the
  releases URL and dismisses the modal.

Dependency: adds `ureq = { version = "2", default-features = false,
features = ["tls"] }` — the smallest blocking HTTP client + TLS combo
that doesn't drag in an async runtime.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/update.rs

@@ -974,6 +974,9 @@ impl H7CAD {
                 if self.about_window == Some(id) {
                     self.about_window = None;
                 }
+                if self.update_notice_window == Some(id) {
+                    self.update_notice_window = None;
+                }
                 Task::none()
             }
 
@@ -3536,6 +3539,37 @@ impl H7CAD {
                     Task::none()
                 }
             }
+            Message::UpdateCheckResult(latest) => {
+                let Some(version) = latest else {
+                    return Task::none();
+                };
+                self.update_notice_version = Some(version);
+                if let Some(id) = self.update_notice_window {
+                    return window::gain_focus(id);
+                }
+                let (id, task) = window::open(window::Settings {
+                    size: iced::Size::new(480.0, 240.0),
+                    resizable: false,
+                    ..Default::default()
+                });
+                self.update_notice_window = Some(id);
+                task.map(|_| Message::Noop)
+            }
+            Message::UpdateNoticeClose => {
+                if let Some(id) = self.update_notice_window.take() {
+                    window::close(id)
+                } else {
+                    Task::none()
+                }
+            }
+            Message::UpdateNoticeOpenRelease => {
+                let _ = open::that(crate::update_check::RELEASES_PAGE);
+                if let Some(id) = self.update_notice_window.take() {
+                    window::close(id)
+                } else {
+                    Task::none()
+                }
+            }
             Message::PageSetupWidthEdit(s) => {
                 self.page_setup_w = s;
                 Task::none()