Add Excel download feature to historical data page

Students can now download historical data as an Excel file for analysis.

Features:
- Download button on historical data page
- Excel file contains 4 sheets:
  1. Summary - Watch model information
  2. Monthly Data - All 120 months of detailed data (demand, revenue, profit)
  3. Yearly Summary - 10 years of aggregated data
  4. Analysis Template - Pre-formatted sheet for Year 11 predictions
- Styled headers with color coding
- Proper column widths for readability
- Filename includes current date

Dependencies:
- Added openpyxl for Excel file generation

This allows students to perform their own analysis in Excel, create charts,
and use formulas before submitting their predictions in the web app.

🤖 Generated with [Claude Code](https://claude.com/claude-code

)

Co-Authored-By: Claude <noreply@anthropic.com>