After saving an expense, checks if the category budget reaches 75%,
90%, or 100% thresholds. Sends gio::Notification for each newly
crossed threshold. Tracks sent notifications in database to prevent
duplicates within the same month.
Currency dropdown now uses 30 currencies from ExchangeRateService.
Defaults to base currency from settings. Selecting a different
currency fetches the exchange rate asynchronously and displays it.
Rate is stored with the transaction on save.
Initialize SQLite database at XDG data directory on app startup.
Categories now load from database with emoji icons. Save button
validates input, inserts transaction, shows toast notification,
clears the form, and refreshes the recent transactions list.
