feat: enhance CI workflow and update webapp styles

- Added Node.js setup and webapp testing steps to the CI workflow for improved integration.
- Updated HTML to link multiple CSS files for better modularity and organization of styles.
- Removed deprecated `style.css` and introduced new CSS files for base styles, calendar, day detail, hints, markers, states, and duty list to enhance maintainability and readability.
- Implemented new styles for improved presentation of duty information and user interactions.
- Added unit tests for new API functions and contact link rendering to ensure functionality and reliability.

webapp/js/dateUtils.test.js

@@ -10,6 +10,10 @@ import {
   dutyOverlapsLocalRange,
   getMonday,
   formatHHMM,
+  firstDayOfMonth,
+  lastDayOfMonth,
+  formatDateKey,
+  dateKeyToDDMM,
 } from "./dateUtils.js";
 
 describe("localDateString", () => {
@@ -157,3 +161,70 @@ describe("formatHHMM", () => {
     expect(result).toMatch(/^\d{2}:\d{2}$/);
   });
 });
+
+describe("firstDayOfMonth", () => {
+  it("returns first day of month", () => {
+    const d = new Date(2025, 5, 15);
+    const result = firstDayOfMonth(d);
+    expect(result.getFullYear()).toBe(2025);
+    expect(result.getMonth()).toBe(5);
+    expect(result.getDate()).toBe(1);
+  });
+
+  it("handles January", () => {
+    const d = new Date(2025, 0, 31);
+    const result = firstDayOfMonth(d);
+    expect(result.getDate()).toBe(1);
+    expect(result.getMonth()).toBe(0);
+  });
+});
+
+describe("lastDayOfMonth", () => {
+  it("returns last day of month", () => {
+    const d = new Date(2025, 0, 15);
+    const result = lastDayOfMonth(d);
+    expect(result.getFullYear()).toBe(2025);
+    expect(result.getMonth()).toBe(0);
+    expect(result.getDate()).toBe(31);
+  });
+
+  it("returns 28 for non-leap February", () => {
+    const d = new Date(2023, 1, 1);
+    const result = lastDayOfMonth(d);
+    expect(result.getDate()).toBe(28);
+    expect(result.getMonth()).toBe(1);
+  });
+
+  it("returns 29 for leap February", () => {
+    const d = new Date(2024, 1, 1);
+    const result = lastDayOfMonth(d);
+    expect(result.getDate()).toBe(29);
+  });
+});
+
+describe("formatDateKey", () => {
+  it("formats ISO date string as DD.MM (local time)", () => {
+    const result = formatDateKey("2025-02-25T00:00:00Z");
+    expect(result).toMatch(/^\d{2}\.\d{2}$/);
+    const [day, month] = result.split(".");
+    expect(Number(day)).toBeGreaterThanOrEqual(1);
+    expect(Number(day)).toBeLessThanOrEqual(31);
+    expect(Number(month)).toBeGreaterThanOrEqual(1);
+    expect(Number(month)).toBeLessThanOrEqual(12);
+  });
+
+  it("returns DD.MM format with zero-padding", () => {
+    const result = formatDateKey("2025-01-05T12:00:00Z");
+    expect(result).toMatch(/^\d{2}\.\d{2}$/);
+  });
+});
+
+describe("dateKeyToDDMM", () => {
+  it("converts YYYY-MM-DD to DD.MM", () => {
+    expect(dateKeyToDDMM("2025-02-25")).toBe("25.02");
+  });
+
+  it("handles single-digit day and month", () => {
+    expect(dateKeyToDDMM("2025-01-09")).toBe("09.01");
+  });
+});