feat: plan Today by available time (#459)

frontend/index.html

@@ -180,6 +180,9 @@
       <button id="cancel-plan-today" type="button">Cancel</button>
     </div>
     <div class="plan-today-capacity" id="plan-today-capacity" role="status" aria-live="polite">0 of 5 selected</div>
+    <label class="plan-today-available" for="plan-today-available">Available today
+      <span><input id="plan-today-available" type="number" inputmode="numeric" min="15" max="1440" step="15" placeholder="Minutes" /> min</span>
+    </label>
     <div class="small plan-today-error" id="plan-today-error" role="alert"></div>
     <section aria-labelledby="today-plan-heading">
       <h3 id="today-plan-heading">Today, in order</h3>