ref(snapshots): two small UI tweaks (#112631)

Diff percent formatting: >= 1% shows 1 decimal (e.g. `93.5%`), < 1%
shows up to 4 decimals without trailing zeros (e.g. `0.0227%`).

Default overlay color changed to red (5th-from-last in palette).

### Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated
in the State of Delaware in 2015 as Functional Software, Inc. and is
gonna need some rights from me in order to utilize my contributions in
this here PR. So here's the deal: I retain all rights, title and
interest in and to my contributions, and by keeping this boilerplate
intact I confirm that Sentry can use, modify, copy, and redistribute my
contributions, under Sentry's choice of terms.

Author: NicoHinderling

static/app/views/preprod/snapshots/main/imageDisplay/diffImageDisplay.tsx

@@ -87,7 +87,12 @@ export function DiffImageDisplay({
     };
   }, [diffImageUrl]);
 
-  const diffPercent = pair.diff === null ? null : `${(pair.diff * 100).toFixed(1)}%`;
+  // >= 1% shows 1 decimal (e.g. "93.5%"), < 1% shows up to 4 without trailing zeros (e.g. "0.0227%")
+  const diffPct = pair.diff === null ? null : pair.diff * 100;
+  const diffPercent =
+    diffPct === null
+      ? null
+      : `${diffPct >= 1 ? diffPct.toFixed(1) : parseFloat(diffPct.toFixed(4))}%`;
 
   return (
     <Flex direction="column" gap="lg" padding="xl" flex="1" minHeight="0">

static/app/views/preprod/snapshots/snapshots.tsx

@@ -76,7 +76,7 @@ export default function SnapshotsPage() {
   const [showOverlay, setShowOverlay] = useState(true);
   const [overlayColor, setOverlayColor] = useState<string>(() => {
     const palette = theme.chart.getColorPalette(10);
-    return palette.at(-1) ?? '#67C800';
+    return palette.at(-5) ?? palette[0];
   });
   const [diffMode, setDiffMode] = useState<DiffMode>('split');