style: apply cargo fmt across the crate (A20)

The repo never enforced rustfmt, so formatting had drifted broadly. This is a
single mechanical `cargo fmt` pass over the whole crate (no behavioral change;
lib suite green, 493 passed). Going forward fmt should be enforced (planned CI
fmt --check step). Part of the 0.6.1 hygiene pass.

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

src/background.rs

@@ -66,7 +66,10 @@ pub fn game_background_filename(game_id: &str) -> String {
 /// recedes the image so body text and panel chrome stay readable, and it re-tints
 /// per theme since `base` comes from the active palette.
 pub fn scrim_color(base: Color, dim: f32) -> Color {
-    Color { a: dim.clamp(0.0, 1.0), ..base }
+    Color {
+        a: dim.clamp(0.0, 1.0),
+        ..base
+    }
 }
 
 #[cfg(test)]