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/theme.rs

@@ -211,7 +211,7 @@ impl AppTheme {
                 overlay: hex(0x6272a4),
                 text: hex(0xf8f8f2),
                 subtext: hex(0xbdc0d4),
-                blue: hex(0xbd93f9),  // Dracula's signature purple as the primary accent
+                blue: hex(0xbd93f9), // Dracula's signature purple as the primary accent
                 lavender: hex(0x8be9fd), // cyan
                 red: hex(0xff5555),
                 maroon: hex(0xff79c6), // pink
@@ -400,7 +400,11 @@ mod tests {
         for theme in AppTheme::ALL {
             let p = theme.palette();
             let sub = contrast_ratio(p.subtext, p.base);
-            assert!(sub >= 3.0, "{}: subtext contrast {sub:.2} < 3.0", theme.label());
+            assert!(
+                sub >= 3.0,
+                "{}: subtext contrast {sub:.2} < 3.0",
+                theme.label()
+            );
             let accent = contrast_ratio(p.blue, p.base);
             assert!(
                 accent >= 3.0,