fix: handle non-JSON error responses in design pipeline

Frontend was crashing with "Unexpected token 'I'" when the backend
returned plain text errors (e.g. "Internal Server Error" from proxy).
Now safely falls back to response.text() when JSON parsing fails.
Also prevents backend from swallowing HTTPException in catch-all.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/design_generator.py

@@ -126,6 +126,8 @@ Square format, clean edges for die-cut sticker."""
             status_code=504,
             detail="AI generation timed out"
         )
+    except HTTPException:
+        raise
     except Exception as e:
         raise HTTPException(
             status_code=502,

frontend/app/design/page.tsx

@@ -54,8 +54,15 @@ export default function DesignPage() {
       });
 
       if (!response.ok) {
+        let message = "Failed to generate design";
+        try {
           const data = await response.json();
-        throw new Error(data.detail || "Failed to generate design");
+          message = data.detail || message;
+        } catch {
+          const text = await response.text();
+          message = text || `Server error (${response.status})`;
+        }
+        throw new Error(message);
       }
 
       const design = await response.json();
@@ -82,8 +89,15 @@ export default function DesignPage() {
       );
 
       if (!response.ok) {
+        let message = "Failed to activate design";
+        try {
           const data = await response.json();
-        throw new Error(data.detail || "Failed to activate design");
+          message = data.detail || message;
+        } catch {
+          const text = await response.text();
+          message = text || `Server error (${response.status})`;
+        }
+        throw new Error(message);
       }
 
       setGeneratedDesign({ ...generatedDesign, status: "active" });
@@ -106,8 +120,15 @@ export default function DesignPage() {
       );
 
       if (!response.ok) {
+        let message = "Failed to delete design";
+        try {
           const data = await response.json();
-        throw new Error(data.detail || "Failed to delete design");
+          message = data.detail || message;
+        } catch {
+          const text = await response.text();
+          message = text || `Server error (${response.status})`;
+        }
+        throw new Error(message);
       }
 
       setGeneratedDesign(null);

frontend/app/upload/page.tsx

@@ -103,8 +103,15 @@ export default function UploadPage() {
       });
 
       if (!response.ok) {
+        let message = "Upload failed";
+        try {
           const data = await response.json();
-        throw new Error(data.detail || "Upload failed");
+          message = data.detail || message;
+        } catch {
+          const text = await response.text();
+          message = text || `Server error (${response.status})`;
+        }
+        throw new Error(message);
       }
 
       const design = await response.json();
@@ -127,8 +134,15 @@ export default function UploadPage() {
         { method: "POST" }
       );
       if (!response.ok) {
+        let message = "Failed to activate design";
+        try {
           const data = await response.json();
-        throw new Error(data.detail || "Failed to activate design");
+          message = data.detail || message;
+        } catch {
+          const text = await response.text();
+          message = text || `Server error (${response.status})`;
+        }
+        throw new Error(message);
       }
       setUploadedDesign({ ...uploadedDesign, status: "active" });
     } catch (err) {
@@ -146,8 +160,15 @@ export default function UploadPage() {
         { method: "DELETE" }
       );
       if (!response.ok) {
+        let message = "Failed to delete design";
+        try {
           const data = await response.json();
-        throw new Error(data.detail || "Failed to delete design");
+          message = data.detail || message;
+        } catch {
+          const text = await response.text();
+          message = text || `Server error (${response.status})`;
+        }
+        throw new Error(message);
       }
       resetForm();
     } catch (err) {