feat(mobile): compare table scroll fade and native share sheet

MOB-14: PerformanceChart and ComparisonChart both already configure
legend position 'top' — no change needed.

MOB-15: Compare's detailedTable is 774px wide inside an overflow-x:
auto wrapper. Add a right-edge mask-image fade at ≤640px so phone
users see the table extends past the viewport.

MOB-16: ComparisonView's Share button previously did clipboard-only.
Prefer navigator.share when available (iOS/Android native sheet) so
users can send straight to Messages / WhatsApp / Mail / etc. Fall
back to clipboard with the existing "Copied!" toast otherwise. User
cancellations swallow silently; only real errors trigger fallback.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nextjs-app/components/ComparisonView.tsx

@@ -157,8 +157,29 @@ export function ComparisonView({
   };
 
   const handleShare = async () => {
+    const url = window.location.href;
+    const count = selectedSchools.length;
+    const shareData = {
+      title: 'School comparison · SchoolCompare',
+      text: count > 0
+        ? `Comparing ${count} school${count === 1 ? '' : 's'} on SchoolCompare`
+        : 'SchoolCompare',
+      url,
+    };
+    // Prefer the native share sheet on platforms that support it (iOS / Android).
+    // canShare is feature-detected because Safari iOS exposes share() but
+    // some configurations refuse the payload.
+    if (typeof navigator !== 'undefined' && navigator.share && (!navigator.canShare || navigator.canShare(shareData))) {
+      try {
+        await navigator.share(shareData);
+        return;
+      } catch (err) {
+        // User cancelled — bail silently. Any other error falls through to clipboard.
+        if ((err as DOMException)?.name === 'AbortError') return;
+      }
+    }
     try {
-      await navigator.clipboard.writeText(window.location.href);
+      await navigator.clipboard.writeText(url);
       setShareConfirm(true);
       setTimeout(() => setShareConfirm(false), 2000);
     } catch { /* fallback: do nothing */ }