Redesign BP tracker with lightweight views and unobtrusive save confirmations

app/templates/error.html

@@ -0,0 +1,3 @@
+{% extends '_layout.html' %}
+{% block title %}Choose a valid period · BP Tracker{% endblock %}
+{% block content %}<section class="card form-page"><h1>Check your dates</h1><p class="field-error" role="alert">{{ message }}</p><a class="button" href="{{ url_for('main.dashboard') }}">Back to overview</a></section>{% endblock %}