fix: allow replacing a model's only file in the edit dialog

Saving deleted removed files before uploading new ones, so swapping a model's
only file failed with "Cannot remove the only plate of a print". New files are
now uploaded first, then removed ones deleted, and a replacement that got a
" (2)" suffix while the old file existed is renamed back to its own name.

backend/tests/api.test.ts

@@ -273,6 +273,25 @@ describe("plate add/remove/reorder/rename", () => {
     res = await auth(request(app).delete(`/api/print/${printId}/plates/${lastPlateId}`));
     expect(res.status).toBe(409);
   });
+
+  // The Edit dialog's order for swapping a model's only file for an edited copy of the same name.
+  it("replaces the only plate with a same-named file: upload, delete the old one, rename back", async () => {
+    let res = await auth(request(app).get(`/api/print/${printId}`));
+    const [original] = res.body.plates;
+
+    res = await auth(request(app).post(`/api/print/${printId}/plates`)).attach(
+      "files",
+      tmpFile(original.filename, "solid replacement endsolid"),
+    );
+    const added = res.body.print.plates.find((p: any) => p.id !== original.id);
+    expect(added.filename).not.toBe(original.filename); // suffixed while the old one exists
+
+    res = await auth(request(app).delete(`/api/print/${printId}/plates/${original.id}`));
+    expect(res.status).toBe(200);
+    res = await auth(request(app).post(`/api/print/${printId}/plate/${added.id}/rename`)).send({ filename: original.filename });
+    expect(res.status).toBe(200);
+    expect(res.body.print.plates).toEqual([expect.objectContaining({ id: added.id, filename: original.filename })]);
+  });
 });
 
 describe("category CRUD + cycle rejection", () => {

frontend/src/pages/ModelDetailPage/EditModelModal.tsx

@@ -247,26 +247,37 @@ export default function EditModelModal({ print, onClose, onUnauthorized, onUpdat
         latest = reorderRes.print ?? latest;
       }
 
-      // Model files (plates): same delete -> upload -> reorder sequence. Always at least one
-      // plate stays staged locally (removePlateItem refuses to drop the last one), matching the
-      // backend's own "can't remove the only plate" rule.
-      const keptPlateIds = new Set(plateItems.filter((p) => p.kind === "existing").map((p) => p.id));
-      for (const original of print.plates) {
-        if (keptPlateIds.has(original.id)) continue;
-        const res = await printsApi.deletePlate(print.id, original.id);
-        latest = res.print ?? latest;
-      }
+      // Model files (plates): upload -> delete -> rename -> reorder. Uploading first is what lets
+      // a model's only file be swapped for a new one in a single save -- deleting first would
+      // briefly leave it with none, which the backend refuses ("can't remove the only plate").
       const newPlateFiles = plateItems.filter((p) => p.kind === "new").map((p) => p.file);
       let newPlateIdsInOrder: string[] = [];
       if (newPlateFiles.length) {
         const beforeIds = new Set(latest.plates.map((p) => p.id));
         const uploadRes = await printsApi.addPlates(print.id, newPlateFiles);
         latest = uploadRes.print;
+        // The backend appends them in upload order, so this lines up with newPlateFiles.
         newPlateIdsInOrder = latest.plates
           .filter((p) => !beforeIds.has(p.id))
           .toSorted((a, b) => a.position - b.position)
           .map((p) => p.id);
       }
+      const keptPlateIds = new Set(plateItems.filter((p) => p.kind === "existing").map((p) => p.id));
+      for (const original of print.plates) {
+        if (keptPlateIds.has(original.id)) continue;
+        const res = await printsApi.deletePlate(print.id, original.id);
+        latest = res.print ?? latest;
+      }
+      // A replacement usually has the same name as the file it replaces, and while that one still
+      // existed the backend gave it a " (2)" suffix. Now it's gone, ask for the name as uploaded:
+      // the backend keeps the suffix if another of the model's files still has that name.
+      for (const [index, plateId] of newPlateIdsInOrder.entries()) {
+        const wanted = newPlateFiles[index].name;
+        const current = latest.plates.find((p) => p.id === plateId);
+        if (!current || current.filename === wanted) continue;
+        const res = await printsApi.renamePlate(print.id, plateId, wanted);
+        latest = res.print ?? latest;
+      }
       {
         let nextNew = 0;
         const finalOrder = plateItems.map((p) => (p.kind === "existing" ? p.id : newPlateIdsInOrder[nextNew++]));