Keep the test scene's own arithmetic from overflowing a u64

The first compile this branch ever had. `cargo fmt` reflowed four files
and clippy passed at -D warnings untouched, but one test panicked:
`the_signature_does_not_change_with_scale`, on "attempt to multiply with
overflow".

It is the fixture, not the feature. `scene()`'s little LCG multiplied the
block's y by the golden-ratio constant with a plain `*` while the term
beside it already used `wrapping_mul`, so any scene taller than about 104
pixels overflowed in debug. Only the scale test builds one that large,
which is why 345 of 346 passed around it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/library_ui.rs

@@ -3790,7 +3790,9 @@ fn start_thumbnail_sweep(window: &AppWindow, ctl: &Rc<LibraryController>) {
                                 library::catalog_path(&conn.account),
                                 library::thumbs_dir(&conn.account),
                                 move |bursts| {
-                                    let Some(w) = weak_after.upgrade() else { return };
+                                    let Some(w) = weak_after.upgrade() else {
+                                        return;
+                                    };
                                     if bursts > 0 && w.get_show_library() {
                                         schedule_reload(&w, &ctl_after);
                                     }