Enhance web UI with improved explanations and UX elements

[prasham21]

This PR works on the enhancement of user experience on the web app. It deals with these:
Homepage:

• Add section titles, blurbs, and improve typography

Forms:

• Replace checkbox with Bootstrap toggle switch for INDRA path analysis
• Increase background genes textarea size to 6 rows
• Enhance loading indicator with dotted spinner and status text

Results Pages:

• Add info icons with Bootstrap popovers explaining CURIE, p/q-values, and statements
• Add overlap column showing contributing genes with expandable lists (discrete analysis)
• Add visual separator between regular and INDRA path-based analysis tabs
• Add ranking explanations clarifying statistical significance

Backend:

• Add overlap_count and overlap_genes columns to ORA results for gene mapping display