Month navigation with prev/next buttons, transactions grouped by date
with headers (Today/Yesterday/formatted date), category icons, amounts
color-coded green for income and red for expenses, and daily net totals.
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.
AdwNavigationSplitView with six sidebar items (Log, History, Charts,
Budgets, Recurring, Settings) and placeholder StatusPage content that
switches via crossfade transition on selection.