feat: add category spending bar chart to Statistics tab

[7174Andy]

Summary

• Adds a horizontal bar chart showing spending by category to the Statistics tab, displayed below the existing metric cards
• Makes the Statistics tab scrollable so all content remains accessible as more visualizations are added
• Bar chart dynamically sizes to fit its content (number of categories) rather than stretching to fill space

Details

Repository layer

• New get_spending_by_category(start_date, end_date) method on TransactionRepository — same query as get_top_spending_category but returns all categories instead of LIMIT 1, sorted descending by total spending

Service layer

• New get_monthly_category_breakdown(year, month) on StatisticsService — delegates to repository after converting year/month to date range via existing _get_month_date_range()

GUI

• Statistics tab content is now wrapped in a scrollable Canvas with mousewheel support (handles both macOS and Windows delta conventions)
• Bar chart section below cards: colored horizontal bars proportional to max spending, category labels on the left, dollar amounts on the right
• 10-color palette cycling for distinct bars
• Empty state shows "No expenses this month" centered text
• Chart redraws on month navigation and tab refresh

Tests

• 4 repository tests: multiple categories sorted descending, empty, income-only, date range filtering
• 3 service tests: with data, empty, December year boundary edge case

Design decisions

• Native Canvas drawing (no external chart libs) — consistent with the existing HeatmapTab pattern
• Content-sized chart — canvas height is computed from num_categories * (bar_height + gap) so the box fits its content exactly
• Platform-aware scrolling — macOS event.delta is small (1/-1) while Windows uses multiples of 120; handled via platform.system() check

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