fix(auto-update): label same-tag rebuilds as 'Rebuild available' instead of '10.11 -> 10.11' (#766)

When a registry pushes a new build of an image at the same tag (digest
changes, tag does not), the preview service set next_tag to the same
string as current_tag and the readiness view rendered '10.11 -> 10.11',
which reads as a UI bug.

Add an update_kind field to UpdatePreviewSummary that distinguishes:
- 'tag'    - at least one image has a strictly newer tag
- 'digest' - the only updates available are same-tag rebuilds
- 'none'   - nothing to apply

The frontend now branches on update_kind and renders 'Rebuild available'
next to the current tag for the digest case, leaving the version-arrow
diff for genuine tag bumps.

Three new buildSummary cases lock in the kind classification.

frontend/src/components/AutoUpdateReadinessView.tsx

@@ -19,6 +19,8 @@ interface UpdatePreviewImage {
   semver_bump: SemverBump;
 }
 
+type UpdateKind = 'tag' | 'digest' | 'none';
+
 interface UpdatePreview {
   stack_name: string;
   images: UpdatePreviewImage[];
@@ -28,6 +30,7 @@ interface UpdatePreview {
     current_tag: string | null;
     next_tag: string | null;
     semver_bump: SemverBump;
+    update_kind: UpdateKind;
     blocked: boolean;
     blocked_reason: string | null;
   };
@@ -160,10 +163,19 @@ function StackReadinessCard({
           const blockedReason = p.summary.blocked_reason;
           return (
             <>
-              <VersionDiff
-                current={p.summary.current_tag}
-                next={p.summary.next_tag}
-              />
+              {p.summary.update_kind === 'digest' ? (
+                <div className="flex items-baseline gap-2 font-mono text-sm">
+                  <span className="text-stat-subtitle">{p.summary.current_tag}</span>
+                  <span className="text-brand text-[11px] uppercase tracking-[0.16em]">
+                    Rebuild available
+                  </span>
+                </div>
+              ) : (
+                <VersionDiff
+                  current={p.summary.current_tag}
+                  next={p.summary.next_tag}
+                />
+              )}
 
               <div className="flex items-center gap-1.5 font-mono text-[11px] text-stat-subtitle/80">
                 <span>{p.summary.primary_image ?? '-'}</span>