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/view.rs

@@ -190,6 +190,13 @@ impl H7CAD {
         if Some(window_id) == self.about_window {
             return crate::ui::about::view_window();
         }
+        if Some(window_id) == self.update_notice_window {
+            let latest = self
+                .update_notice_version
+                .as_deref()
+                .unwrap_or("?");
+            return crate::ui::update_notice::view_window(latest);
+        }
         if Some(window_id) == self.unsaved_dialog_window {
             let tab_name = match &self.pending_close {
                 Some(super::PendingClose::Tab(idx)) => self