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>

src/components/calendar.rs

@@ -67,13 +67,25 @@ pub fn Calendar(props: &CalendarProps) -> Html {
             current_month.set(next);
         })
     };
+
+    let go_to_today = {
+        let current_month = current_month.clone();
+        Callback::from(move |_| {
+            let today = Local::now().date_naive();
+            let first_of_today_month = today.with_day(1).unwrap();
+            current_month.set(first_of_today_month);
+        })
+    };
     
     html! {
         <div class="calendar">
             <div class="calendar-header">
                 <button class="nav-button" onclick={prev_month}>{"‹"}</button>
                 <h2 class="month-year">{format!("{} {}", get_month_name(current_month.month()), current_month.year())}</h2>
-                <button class="nav-button" onclick={next_month}>{"›"}</button>
+                <div class="header-right">
+                    <button class="today-button" onclick={go_to_today}>{"Today"}</button>
+                    <button class="nav-button" onclick={next_month}>{"›"}</button>
+                </div>
             </div>
             
             <div class="calendar-grid">