REFACTOR(arch): Transition to multi-page Streamlit app

Refactored the application architecture from a single monolithic script
into a multi-page structure, adhering to Streamlit best practices.

- Renamed the main `app.py` to `1_🔬_Upload_and_Run.py` to serve as the
primary landing and inference page.
- This structural change enables the creation of dedicated pages for
specialized functionality, such as the new analysis dashboard,
improving code organization and user navigation.

1_Upload_and_Run.py

@@ -0,0 +1,41 @@
+"""Streamlit main entrance; modularized for clarity"""
+
+import streamlit as st
+
+from modules.callbacks import init_session_state
+
+from modules.ui_components import (
+    render_sidebar,
+    render_results_column,
+    render_input_column,
+    load_css,
+)
+
+
+# --- Page Setup (Called only ONCE) ---
+st.set_page_config(
+    page_title="ML Polymer Classification",
+    page_icon="🔬",
+    layout="wide",
+    initial_sidebar_state="expanded",
+    menu_items={"Get help": "https://github.com/KLab-AI3/ml-polymer-recycling"},
+)
+
+
+def main():
+    """Modularized main content to other scripts to clean the main app"""
+    load_css("static/style.css")
+    init_session_state()
+
+    # Render UI components
+    render_sidebar()
+
+    col1, col2 = st.columns([1, 1.35], gap="small")
+    with col1:
+        render_input_column()
+    with col2:
+        render_results_column()
+
+
+if __name__ == "__main__":
+    main()