refactor(app): Extract PDF conversion logic into a separate module

The main.py script was becoming monolithic, containing all the logic for PDF conversion, image path simplification, and content refinement. This change extracts these core functionalities into a new `pdf_convertor` module.

This refactoring improves the project structure by:
- Enhancing modularity and separation of concerns.
- Making the main.py script a cleaner, high-level orchestrator.
- Improving code readability and maintainability.

The functions `convert_pdf_to_markdown`, `save_md_images`, and `refine_content` are now imported from the `pdf_convertor` module and called from the main execution block.

llm.py

@@ -0,0 +1,23 @@
+import configparser
+import os
+
+
+def set_gemini_api_key() -> None:
+    config = configparser.ConfigParser()
+    config.read("config.ini")
+    google_api_key = config.get("llm", "GOOGLE_API_KEY", fallback=None)
+
+    if not os.environ.get("GOOGLE_API_KEY"):
+        if google_api_key:
+            os.environ["GOOGLE_API_KEY"] = google_api_key
+        else:
+            raise ValueError(
+                "Error: GOOGLE_API_KEY not found in config.ini or environment variables"
+            )
+    return
+
+
+def get_model_name() -> str:
+    config = configparser.ConfigParser()
+    config.read("config.ini")
+    return config.get("llm", "MODEL_NAME", fallback="gemini-2.5-flash")