Feat: Fix to Yearly Heat Map Instead of Monthly Heat Map

[7174Andy]

Summary

• Redesign the Heatmap tab from a monthly calendar grid to a GitHub-style yearly heatmap, showing all 365/366 days of a year at a glance with green color intensity based on spending levels
• Add yearly summary cards below the heatmap displaying Total Spent, Daily Average, and Busiest Day
• Fix tab refresh so that all tabs (not just Transactions) refresh after a dialog closes (e.g. PDF import), ensuring Statistics and Heatmap show up-to-date data immediately

Changes

Heatmap Tab (heatmap_tab.py) — Full Rewrite

• Replaced the monthly calendar grid with a compact yearly heatmap (7 rows x 53 columns)
• 5-level green color palette (#ebedf0 to #216e39) with quartile-based thresholds
• Day-of-week labels (Mon/Wed/Fri) on the left, month abbreviations along the top
• Hover tooltips showing exact spending and date
• Click-to-filter: clicking a day cell navigates to the Transactions tab filtered to that date
• Less/More legend at bottom-right
• Year navigation (< >) to browse years with expense data
• Three summary cards below the heatmap: Total Spent, Daily Average, Busiest Day

Repository (transaction_repository.py)

• get_daily_spending_for_year(year) — returns daily spending totals keyed by ISO date string
• get_years_with_expenses() — returns sorted descending list of years that have expense data

Statistics Service (statistics.py)

• get_yearly_heatmap_data(year) — delegates to the new repository method
• get_available_years() — delegates to the new repository method

Main Window (main_window.py)

• Extracted _refresh_active_tab() to refresh whichever tab is currently visible
• Dialog close handler now calls _refresh_active_tab() instead of hardcoded transactions_tab.refresh()
• _on_tab_changed() simplified to delegate to _refresh_active_tab()

Test Plan

• Unit tests for get_daily_spending_for_year() and get_years_with_expenses() in tests/core/test_repository.py
• Unit tests for get_yearly_heatmap_data() and get_available_years() in tests/services/test_statistics.py
• Manual: launch app, verify heatmap renders correctly with year navigation
• Manual: hover over cells to see tooltips, click cells to filter transactions
• Manual: import a PDF while on Statistics/Heatmap tab, verify data refreshes without needing to switch tabs