Add 'Today' button to calendar header for quick navigation to current month

- Add go_to_today callback that navigates to first day of current month
- Position Today button between month title and next month arrow
- Center month/year title using absolute positioning
- Group Today button and next arrow in header-right container with 0.5rem gap
- Style Today button with pill shape, semi-transparent background, and hover effects
- Add responsive styling for mobile screens with smaller text and padding
- Maintain clean, balanced header layout on all screen sizes

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

styles.css

@@ -405,6 +405,15 @@ body {
     font-size: 1.8rem;
     font-weight: 600;
     margin: 0;
+    position: absolute;
+    left: 50%;
+    transform: translateX(-50%);
+}
+
+.header-right {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
 }
 
 .nav-button {
@@ -427,6 +436,25 @@ body {
     background: rgba(255,255,255,0.3);
 }
 
+.today-button {
+    background: rgba(255,255,255,0.2);
+    border: none;
+    color: white;
+    font-size: 0.9rem;
+    font-weight: 500;
+    padding: 0.5rem 1rem;
+    border-radius: 20px;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    transition: background-color 0.2s;
+}
+
+.today-button:hover {
+    background: rgba(255,255,255,0.3);
+}
+
 .calendar-grid {
     display: grid;
     grid-template-columns: repeat(7, 1fr);
@@ -765,6 +793,11 @@ body {
         height: 35px;
         font-size: 1.2rem;
     }
+    
+    .today-button {
+        font-size: 0.8rem;
+        padding: 0.4rem 0.8rem;
+    }
 
     .weekday-header {
         padding: 0.5rem;