fix: normalize aspect ratio per model, surface invalid_params in errors

Models like mystic, flux-pro-1.1, and seedream-v4/v4-5 require named
aspect ratio slugs (e.g. "square_1_1", "widescreen_16_9") while other
models accept the "W:H" format directly.

- Add normalize_aspect_ratio() mapping W:H strings to slugs for affected models
- Apply normalization in generate-image before building the request payload
- Improve FreepikAPIError to surface invalid_params field details from the API
  response, so "Validation error" now also shows which field failed and why

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

freepik_cli/api/client.py

@@ -34,10 +34,17 @@ class FreepikAPIError(Exception):
             or f"HTTP {response.status_code}"
         )
 
+        # Append individual field validation errors when present
+        invalid = body.get("invalid_params", [])
+        if invalid:
+            details = "\n".join(
+                f"  • {p.get('field', '?')}: {p.get('reason', '')}" for p in invalid
+            )
+            message = f"{message}\n\n{details}"
+
         hints = {
             401: "Check your API key — set FREEPIK_API_KEY or use --api-key.",
             403: "Your plan may not support this feature. Check your Freepik subscription.",
-            422: "Invalid request parameters. Check the options you provided.",
             429: "Rate limit exceeded. Please wait before retrying.",
         }
         hint = hints.get(response.status_code)