fix(recurring): keep start date and allow clearing end date on edit

[robitec97]

🎯 Description

Editing a recurring transaction changed its start date to whatever selectedDateProvider held: today, or the date of the last transaction opened in the add page. That also moved the day of the month future payments are inserted on. Selecting "Never" as end date was ignored, so the old end date was kept.

Closes: #545

📱 Changes

• RecurringTransactionsNotifier.updateTransaction no longer sets fromDate. The start date is read-only on the edit page, so the stored value is kept.
• RecurringTransaction.copy() can now set toDate to null, using the same _unset sentinel Transaction.copy() already uses for id.
• New provider test test/providers/recurring_transactions_provider_test.dart that runs the real edit flow (transactionSelect → updateTransaction) against the test DB. On current main both tests fail:
  • start date: expected 2024-01-01, actual 2025-05-20 (the stale selectedDateProvider value)
  • "Never": expected null, actual 2027-01-01

🧪 Testing Instructions

Behaviour

1. Create a monthly recurring transaction with a start date in the past (e.g. the 1st of last month) and an end date.
2. Open it from Planning, change only the amount, press UPDATE TRANSACTION.
3. Reopen it: "Date Start" is unchanged (before: today).
4. Edit it again, set the end date to "Never", update, reopen: the end date is "Never" (before: the old date came back).

dart format, flutter analyze and flutter test --concurrency=1 all pass locally (Flutter 3.38.8).

🔍 Checklist for reviewers

• Code is formatted correctly
• Tests are passing
• New tests are added (if needed)
• Style matches the figma/designer requests
• Tested on:
  • iOS
  • Android

[mikev-cw]

nice catch, thank you!