Restore the Reapply target when a model load fails to start

A load request that is rejected up front (validation error, gated repo,
training guard) leaves the previously loaded model resident, but both the
Images and Video pages had already pointed lastLoad at the failed pick, so
Reapply and the resident-default seeding retried the wrong model. Snapshot
the prior target before the optimistic assignment and restore it (plus the
video page's canReapply flag) when the start POST rejects; successful loads
and failures after the background load starts behave as before.

studio/frontend/src/features/images/images-page.tsx

@@ -1536,6 +1536,10 @@ export function ImagesPage({ active = true }: { active?: boolean }) {
       lastLoadSig.current = null;
       loadToastId.current = toast(null, loadToastArgs(IDLE_PROGRESS));
       // Remember what was loaded so "Reapply" can reload it with new advanced options.
+      // Snapshot the prior target first: a load that fails to START (validation, gated
+      // repo, training guard) leaves the previous model resident, so Reapply and the
+      // resident-default seeding must keep pointing at it, not at the failed pick.
+      const prevLastLoad = lastLoad.current;
       lastLoad.current = { repoId, kind: opts.kind, filename: opts.filename };
       try {
         // Returns immediately — the load runs in the background; we poll for it.
@@ -1557,6 +1561,7 @@ export function ImagesPage({ active = true }: { active?: boolean }) {
           transformer_cache: transformerCache === "auto" ? undefined : transformerCache,
         });
       } catch (err) {
+        lastLoad.current = prevLastLoad;
         dismissLoadToast();
         toast.error(err instanceof Error ? err.message : "Failed to start load");
         setBusy(null);

studio/frontend/src/features/video/video-page.tsx

@@ -936,6 +936,11 @@ export function VideoPage({ active = true }: { active?: boolean }) {
       dismissLoadToast();
       lastLoadSig.current = null;
       loadToastId.current = toast(null, loadToastArgs(IDLE_PROGRESS));
+      // Snapshot the prior Reapply target first: a load that fails to START (validation,
+      // gated repo, training guard) leaves the previous model resident, so Reapply must
+      // keep pointing at it, not at the failed pick.
+      const prevLastLoad = lastLoad.current;
+      const prevCanReapply = canReapply;
       lastLoad.current = { repoId, kind: opts.kind, filename: opts.filename };
       setCanReapply(true);
       try {
@@ -954,6 +959,8 @@ export function VideoPage({ active = true }: { active?: boolean }) {
           transformer_quant: transformerQuant === "auto" ? undefined : transformerQuant,
         });
       } catch (err) {
+        lastLoad.current = prevLastLoad;
+        setCanReapply(prevCanReapply);
         dismissLoadToast();
         toast.error(err instanceof Error ? err.message : "Failed to start load");
         setBusy(null);
@@ -967,6 +974,7 @@ export function VideoPage({ active = true }: { active?: boolean }) {
       pollLoadProgress,
       refreshStatus,
       dismissLoadToast,
+      canReapply,
       memoryMode,
       speedMode,
       attentionBackend,