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/css/base.css

@@ -0,0 +1,94 @@
+/* === Variables & themes */
+:root {
+  --bg: #1a1b26;
+  --surface: #24283b;
+  --text: #c0caf5;
+  --muted: #565f89;
+  --accent: #7aa2f7;
+  --duty: #9ece6a;
+  --today: #bb9af7;
+  --unavailable: #e0af68;
+  --vacation: #7dcfff;
+  --error: #f7768e;
+  --timeline-date-width: 3.6em;
+  --timeline-track-width: 10px;
+  --transition-fast: 0.15s;
+  --transition-normal: 0.25s;
+  --ease-out: cubic-bezier(0.32, 0.72, 0, 1);
+}
+
+@media (prefers-reduced-motion: reduce) {
+  *,
+  *::before,
+  *::after {
+    animation-duration: 0.01ms !important;
+    animation-iteration-count: 1 !important;
+    transition-duration: 0.01ms !important;
+  }
+}
+
+/* Light theme: prefer Telegram themeParams (--tg-theme-*), fallback to Telegram-like palette */
+[data-theme="light"] {
+  --bg: var(--tg-theme-bg-color, #f0f1f3);
+  --surface: var(--tg-theme-secondary-bg-color, #e0e2e6);
+  --text: var(--tg-theme-text-color, #343b58);
+  --muted: var(--tg-theme-hint-color, #6b7089);
+  --accent: var(--tg-theme-link-color, #2e7de0);
+  --duty: #587d0a;
+  --today: var(--tg-theme-link-color, var(--tg-theme-accent-text-color, #2481cc));
+  --unavailable: #b8860b;
+  --vacation: #0d6b9e;
+  --error: #c43b3b;
+}
+
+/* Dark theme: prefer Telegram themeParams, fallback to Telegram dark palette */
+[data-theme="dark"] {
+  --bg: var(--tg-theme-bg-color, #17212b);
+  --surface: var(--tg-theme-secondary-bg-color, #232e3c);
+  --text: var(--tg-theme-text-color, #f5f5f5);
+  --muted: var(--tg-theme-hint-color, #708499);
+  --accent: var(--tg-theme-link-color, #6ab3f3);
+  --today: var(--tg-theme-link-color, var(--tg-theme-accent-text-color, #6ab2f2));
+  --duty: #5c9b4a;
+  --unavailable: #b8860b;
+  --vacation: #5a9bb8;
+  --error: #e06c75;
+}
+
+/* === Layout & base */
+html {
+  scrollbar-gutter: stable;
+  scrollbar-width: none;
+  -ms-overflow-style: none;
+  overscroll-behavior: none;
+}
+
+html::-webkit-scrollbar {
+  display: none;
+}
+
+* {
+  box-sizing: border-box;
+}
+
+body {
+  margin: 0;
+  padding: 0;
+  font-family: system-ui, -apple-system, sans-serif;
+  background: var(--bg);
+  color: var(--text);
+  min-height: 100vh;
+  -webkit-tap-highlight-color: transparent;
+}
+
+.container {
+  max-width: 420px;
+  margin: 0 auto;
+  padding: 12px;
+  padding-top: 0px;
+  padding-bottom: env(safe-area-inset-bottom, 12px);
+}
+
+[data-theme="light"] .container {
+  border-radius: 12px;
+}