App: JavaScript buttons for switching days

app/templates/main.html

@@ -67,9 +67,11 @@
 	<p id=intro>This table summarizes meetings scheduled in our Zoom accounts. Use standard Zoom interface
 	to create and modify reservations. Please keep in mind that there is a slight delay between Zoom
 	and this table. Please report all bugs to Martin Mareš.</p>
-	<form method=GET action="?">
+	<form id=f method=GET action="?">
 		<label for=date>Date:</label>
-		<input id=date type=date name=date value="{{ g.date }}">
+		<input id=date type=date name=date step=1 value="{{ g.date }}">
+		<button type=button onclick="document.getElementById('date').stepDown(); document.getElementById('f').requestSubmit()">←</button>
+		<button type=button onclick="document.getElementById('date').stepUp();   document.getElementById('f').requestSubmit()">→</button>
 		<select name=hours>
 			<option value=0{{ " selected" if g.hours==0 else "" }}>Study hours</option>
 			<option value=1{{ " selected" if g.hours==1 else "" }}>Working hours</option>