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/commands/generate.py

@@ -10,7 +10,7 @@ import typer
 from freepik_cli.api.client import FreepikAPIError, FreepikClient
 from freepik_cli.api.edit import EditAPI
 from freepik_cli.api.images import ImageAPI
-from freepik_cli.api.models import IconStyle, ImageModel, VideoModel
+from freepik_cli.api.models import IconStyle, ImageModel, VideoModel, normalize_aspect_ratio
 from freepik_cli.api.videos import VideoAPI
 from freepik_cli.utils.config import FreepikConfig
 from freepik_cli.utils.console import GenerationResult, console, print_error, print_no_wait, print_result
@@ -86,7 +86,7 @@ def generate_image(
     # Build request payload
     payload: dict = {"prompt": prompt}
     if aspect_ratio:
-        payload["aspect_ratio"] = aspect_ratio
+        payload["aspect_ratio"] = normalize_aspect_ratio(aspect_ratio, model)
     if negative_prompt:
         payload["negative_prompt"] = negative_prompt
     if seed is not None: