Add CSV/JSON export for tasks and activity log

[MeisnerDan]

Allow users to export their tasks and activity log in CSV or JSON format. This supports data portability, external analysis, and integration with other tools. Exports should include all relevant fields and be structured in a way that's easy to import into spreadsheets or data tools.

Acceptance Criteria

• Export tasks as CSV with columns: ID, Title, Status, Importance, Urgency, Assigned To, Created, Completed
• Export tasks as JSON with full task schema
• Export activity log as CSV with columns: Event Type, Actor, Task ID, Summary, Timestamp
• Export activity log as JSON with full event schema
• Include a date range filter for activity log exports (e.g., "last 30 days")
• Export button in settings or main navigation
• Downloaded files are timestamped (e.g., tasks_2026-02-23.csv)
• Exports handle special characters and escaping correctly
• Unit tests verify CSV/JSON structure and data integrity