feat: add local admin authentication

app/web/templates/login.html

@@ -0,0 +1,16 @@
+{% extends "base.html" %}
+
+{% block title %}Login - MyWhoosh Garmin Sync{% endblock %}
+
+{% block content %}
+<h1>Admin Login</h1>
+{% if error %}
+<p class="error">{{ error }}</p>
+{% endif %}
+<form method="post" action="/login">
+    <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+    <label for="password">Password</label>
+    <input type="password" id="password" name="password" required autofocus>
+    <button type="submit">Log in</button>
+</form>
+{% endblock %}