feat(custom-chips): AI entry point + friendly auth error

- 'Create with AI' button in CustomChipDialog dispatches the generic
  velxio:agent-prompt window event (no-op without a listener — the pro
  overlay's chat panel picks it up and prefills the composer).
- chipCompileService maps the hosted deployment's 401 gate to a human
  'sign in to compile custom chips' message instead of a raw status
  line. Self-hosted OSS keeps the route open and never sees either.

frontend/src/components/customChips/CustomChipDialog.tsx

@@ -283,6 +283,23 @@ export const CustomChipDialog = ({ initial, onClose, onSave }: CustomChipDialogP
               {compiling ? t('editor.customChip.compiling') : t('editor.customChip.compile')}
             </button>
           )}
+          <button
+            style={cancelBtn}
+            title="Describe the chip and the AI agent writes, compiles and programs it"
+            onClick={() => {
+              // Pro AI agent hook — no-op when no overlay listener is mounted.
+              window.dispatchEvent(new CustomEvent('velxio:agent-prompt', {
+                detail: {
+                  prompt:
+                    'Crea un custom chip que ... (describe qué debe hacer; ' +
+                    'el agente escribe el C, lo compila y lo programa en el canvas)',
+                },
+              }));
+              onClose();
+            }}
+          >
+            ✨ Create with AI
+          </button>
           <div style={{ flex: 1 }} />
           <button style={cancelBtn} onClick={onClose}>{t('editor.customChip.cancel')}</button>
           <button style={canSave ? saveBtn : saveBtnDisabled} disabled={!canSave} onClick={doSave}>

frontend/src/services/chipCompileService.ts

@@ -34,7 +34,11 @@ export async function compileChip(source: string, chipJson?: string): Promise<Ch
       wasm_base64: null,
       stdout: '',
       stderr: '',
-      error: `HTTP ${res.status}: ${text}`,
+      // Hosted deployments gate this route behind login (401) — surface a
+      // human instruction instead of a raw status line.
+      error: res.status === 401
+        ? 'login_required: sign in to compile custom chips.'
+        : `HTTP ${res.status}: ${text}`,
       byte_size: 0,
     };
   }