Add default screenshots URL and improve param handling

README.md

@@ -59,7 +59,7 @@ A plane through the point $P$.
 
 ## Parameters for an applet
 
-Some parameters can be set for an applet. Only the `url`, `fig` and `name` parameters are required; the rest is optional. It is recommended to add a `status` to the applet, which can be `unreviewed`, `in-review` or `reviewed`.
+Some parameters can be set for an applet. Only the `url` and `name` parameters are required; the rest is optional. It is recommended to add a `status` to the applet, which can be `unreviewed`, `in-review` or `reviewed`.
 
 ````md
 ```{applet}
@@ -76,11 +76,15 @@ A title that describes the applet
 
 | Parameter                                                                                                                           | Description                                                                                  | Default      |
 | ----------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------- | ------------ |
+| `fig`                                                                                                                               | The path to an image that will be shown in the print version of the document                  | None         |
 | `title`                                                                                                                             | A string that will be shown as the title of the applet when the applet is in fullscreen mode | ""           |
 | `status`                                                                                                                            | The status of the applet. Can be `unreviewed`, `in-review` or `reviewed`                     | `unreviewed` |
 | `width`                                                                                                                             | The width of the applet in pixels                                                            | 100%         |
 | `height`                                                                                                                            | The height of the applet in pixels                                                           | 400px        |
 
+> [!NOTE]
+> If the `fig` parameter is not defined, the code will insert the image provided by the applet itself as a fallback.
+
 ### Control parameters
 
 > [!WARNING]

src/sphinx_prime_applets/__init__.py

@@ -70,8 +70,13 @@ def run(self):
         fig = self.options.get("fig")
 
         assert url is not None
-        assert fig is not None
-
+        if "?" in url:
+             url, url_params = url.split("?", 1)
+        else:
+             url_params = ""
+        if fig is None:
+            fig = DEFAULT_BASE_URL + url + "/image.png"
+
         iframe_class = self.options.get("class")  # expect a list/string of classes
 
         if iframe_class is None:
@@ -86,10 +91,23 @@ def run(self):
         (figure_node,) = Figure.run(self)
 
         # Generate GET params and inline styling
+        # we do not perform validation or sanitization
         params_dict = parse_options(self.options)
         params_dict["iframe"] = (
             "true"  # To let the applet know its being run in an iframe
         )
+        if url_params != "":
+            for param in url_params.split("&"):
+                if "=" in param:
+                    key, value = param.split("=", 1)
+                    params_dict[key] = value
+                else:
+                    params_dict[param] = "true"
+        # overwrite language based on document language
+        lang = self.state.document.settings.env.config.language
+        if lang is None:
+            lang = "en"
+        params_dict["lang"] = lang  # language is always overwritten
         params = "&".join(
             [f"{key}={quote(value)}" for key, value in params_dict.items()]
         )