Moved all activity-related functions from app.py to a new activity.py
module to improve code organization and maintainability. This reduces
app.py from 2852 lines to 1552 lines.
Changes:
- Created activity.py with 16 activity-related functions
- Updated app.py to import and initialize activity module
- Updated test_app.py to import activity module
- All 34 unit tests pass successfully
