Refactor: extract shared code, add TransactionService, consolidate utilities

[7174Andy]

Summary

This PR tackles several architectural issues discovered during a code review, reducing duplication and improving consistency across the codebase:

• Shared expense form builder — Extracted duplicated form construction (~30 identical lines) and amount validation from AddExpenseDialog and EditExpenseDialog into expense_form.py with build_expense_form() and validate_amount()
• TransactionService — Created a centralized service layer that ensures consistent auto-categorization across all entry points (manual add, PDF import, edit). Previously, PDF uploads auto-categorized via MerchantCategoryService but manually added transactions skipped categorization entirely
• Consolidated date parsing — Merged two independent date parsing implementations (UploadDialog._parse_date() and extract._parse_date()) into a single parse_date_from_str() utility in utils/date.py
• Removed duplicated SQL — get_top_spending_category() was nearly identical to get_spending_by_category() (differed only by LIMIT 1). Removed the former; StatisticsService now uses get_spending_by_category()[0]

Key changes

New files

• expense_tracker/gui/dialogs/expense_form.py — Shared form builder and amount validator
• expense_tracker/services/transaction.py — TransactionService with auto-categorization, merchant mapping updates, bulk import with dedup
• expense_tracker/utils/date.py — Consolidated parse_date_from_str() utility
• tests/services/test_transaction.py — 11 tests covering all TransactionService methods

Modified files

• app.py — Creates MerchantCategoryService once, injects into TransactionService
• main_window.py, transactions_tab.py — Accept TransactionService instead of raw repos
• add_expense.py, edit_expense.py, upload.py — Refactored to use shared form + TransactionService
• statistics.py — Uses get_spending_by_category()[0] instead of removed get_top_spending_category()
• transaction_repository.py — Removed get_top_spending_category()
• extract.py — Uses shared parse_date_from_str(), returns date objects instead of strings
• test_extract.py — Updated assertions to expect date objects
• test_repository.py — Removed 4 obsolete tests for deleted get_top_spending_category

Design decisions

• TransactionService wraps MerchantCategoryService rather than exposing it to dialogs directly. This keeps categorization logic centralized — adding a new entry point (e.g., CSV import) only needs to call TransactionService.import_transactions().
• update_transaction() returns bool to indicate whether merchant categories were updated, so the edit dialog can show appropriate feedback without needing to track category changes itself.
• extract.py now returns date objects instead of ISO strings. This is more type-safe and avoids unnecessary string↔date conversions downstream.

Test plan

• All 142 tests pass
• ruff check . clean
• Manual: verify add/edit/upload dialogs still work correctly
• Manual: verify auto-categorization triggers on manual add and PDF import

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