Add prev/next month navigation to Finances date filter

[Copilot]

Users had no way to quickly shift the Finances date filter by month — they had to manually edit both date inputs. This adds ‹ Prev and Next › buttons that snap the date range to the first/last day of the adjacent month and immediately apply the filter.

Changes

• index.html: Added #prevMonthBtn (before the From: input) and #nextMonthBtn (after the This Month button) to the finance filter row.
• src/app.ts:
  • Registered click listeners for both buttons.
  • Added navigateToPrevMonth() and navigateToNextMonth() — each reads the current start date as the base, computes the target month's first/last day, updates both inputs, and calls renderFinances().

navigateToPrevMonth(): void {
    const startInput = document.getElementById('financeStartDate') as HTMLInputElement;
    const base = startInput.value ? new Date(startInput.value + 'T00:00:00') : new Date();
    const firstDay = new Date(base.getFullYear(), base.getMonth() - 1, 1);
    const lastDay = new Date(firstDay.getFullYear(), firstDay.getMonth() + 1, 0);
    startInput.valueAsDate = firstDay;
    (document.getElementById('financeEndDate') as HTMLInputElement).valueAsDate = lastDay;
    this.renderFinances();
}

lastDay is derived from firstDay (not the original base) to correctly handle year boundaries (e.g. Jan → Dec, Dec → Jan). Falls back to current date when no start date is set.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add Monthly Navigation to the Finances filter</issue_title>
<issue_description>A user requested a previous and next month button be added to the Finances page. It should automatically adjust the date filters to be the first of the month and the last of the month and apply the filter.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add Monthly Navigation to the Finances filter #42

---

⌨️ Start Copilot coding agent tasks without leaving your editor — available in VS Code, Visual Studio, JetBrains IDEs and Eclipse.