fix: add inline styles to sr-only spans for CSS-independent accessibility

The sr-only spans now work without requiring the datepicker stylesheet
to be loaded. This fixes an issue where weekday names would show full
names (e.g., "Thursday") instead of abbreviated names when CSS isn't
properly applied - which can happen in certain environments like Tauri.

The accessibility pattern remains the same (screen readers get the full
weekday name, visual users see the abbreviation), but now it works
even when CSS fails to load.

Fixes #6173

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: martijnrusschen

src/calendar.tsx

@@ -509,6 +509,19 @@ export default class Calendar extends Component<CalendarProps, CalendarState> {
       this.props.calendarStartDay,
     );
 
+    // Inline sr-only styles ensure weekday names work without CSS import
+    const srOnlyStyles: React.CSSProperties = {
+      position: "absolute",
+      width: "1px",
+      height: "1px",
+      padding: 0,
+      margin: "-1px",
+      overflow: "hidden",
+      clipPath: "inset(50%)",
+      whiteSpace: "nowrap",
+      border: 0,
+    };
+
     const dayNames: React.ReactElement[] = [];
     if (this.props.showWeekNumbers) {
       dayNames.push(
@@ -517,7 +530,9 @@ export default class Calendar extends Component<CalendarProps, CalendarState> {
           className={`react-datepicker__day-name ${disabled ? "react-datepicker__day-name--disabled" : ""}`}
           role="columnheader"
         >
-          <span className="react-datepicker__sr-only">Week number</span>
+          <span className="react-datepicker__sr-only" style={srOnlyStyles}>
+            Week number
+          </span>
           <span aria-hidden="true">{this.props.weekLabel || "#"}</span>
         </div>,
       );
@@ -568,7 +583,9 @@ export default class Calendar extends Component<CalendarProps, CalendarState> {
               disabled ? "react-datepicker__day-name--disabled" : "",
             )}
           >
-            <span className="react-datepicker__sr-only">{fullDayName}</span>
+            <span className="react-datepicker__sr-only" style={srOnlyStyles}>
+              {fullDayName}
+            </span>
             <span aria-hidden="true">{weekDayName}</span>
           </div>
         );