feat(dashboard): add country distribution bar chart using Chart.js

app/templates/dashboard.html

@@ -59,6 +59,13 @@
 </div>
 {% endif %}
 
+{% if country_stats %}
+<div style="background:var(--card-bg);border:1px solid var(--border);border-radius:10px;padding:1.25rem;margin-bottom:1.5rem">
+    <h2 style="margin-bottom:.75rem">🌍 国家/地区分布</h2>
+    <canvas id="country-chart" height="100"></canvas>
+</div>
+{% endif %}
+
 <div class="dash-sections">
     <div class="dash-section">
         <h2>最近动态</h2>
@@ -129,4 +136,29 @@
     <p class="empty-sub">创建一个 <a href="/profiles">Profile</a> 开始管理你的明信片</p>
 </div>
 {% endif %}
+{% if country_stats %}
+<script src="https://cdn.jsdelivr.net/npm/chart.js@4"></script>
+<script>
+new Chart(document.getElementById('country-chart'), {
+    type: 'bar',
+    data: {
+        labels: [{% for code, _ in country_stats %}'{{ code }}',{% endfor %}],
+        datasets: [{
+            data: [{% for _, count in country_stats %}{{ count }},{% endfor %}],
+            backgroundColor: '#3b82f6',
+            borderRadius: 4,
+            maxBarThickness: 40
+        }]
+    },
+    options: {
+        responsive: true,
+        plugins: { legend: { display: false } },
+        scales: {
+            y: { beginAtZero: true, ticks: { stepSize: 1 } },
+            x: { grid: { display: false } }
+        }
+    }
+});
+</script>
+{% endif %}
 {% endblock %}