perf(scan): replace deep watch with computed to reduce reactivity overhead

During a scan with large collections, the scan page becomes slow or
unresponsive. The watch on scanningPlatforms with { deep: true } was
firing on every ROM addition, causing Vue to deeply traverse all nested
platform and ROM data each time.

The watch's only job is deciding which panels are open. A panel should
open when a platform first receives ROMs — after that, its open/closed
state doesn't need to change regardless of how many more ROMs arrive.

Replace with a computed that maps each platform to a boolean
(roms.length > 0), so the watch only fires on that one meaningful
transition per platform. The ROM list inside each panel continues to
update reactively as normal — this change only affects the panel
open/close logic.

Note: This PR was written with AI assistance (Claude Code).

frontend/src/views/Scan.vue

@@ -90,16 +90,18 @@ watch(metadataOptions, (newOptions) => {
   );
 });
 
-// Adding each new scanned platform to panelIndex to be open by default
-watch(
-  scanningPlatforms,
-  () => {
-    panels.value = scanningPlatforms.value
-      .map((p, index) => (p.roms.length > 0 ? index : -1))
-      .filter((index) => index !== -1);
-  },
-  { deep: true },
+// Track which platforms have ROMs without a deep watch on the entire array.
+// The computed returns a stable string that only changes when a platform
+// transitions from 0→>0 ROMs (or back), so the watch fires O(n_platforms)
+// times rather than O(n_roms) times.
+const platformsWithRomsKey = computed(() =>
+  scanningPlatforms.value.map((p) => (p.roms.length > 0 ? 1 : 0)).join(""),
 );
+watch(platformsWithRomsKey, () => {
+  panels.value = scanningPlatforms.value
+    .map((p, index) => (p.roms.length > 0 ? index : -1))
+    .filter((index) => index !== -1);
+});
 
 const scanOptions = [
   {