revert(dashboard): restore Chart.js bar chart for country distribution

app/templates/dashboard.html

@@ -60,20 +60,33 @@
 {% endif %}
 
 {% if country_stats %}
+<script src="https://cdn.jsdelivr.net/npm/chart.js@4"></script>
 <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>
-    {% set max_count = country_stats[0][1] if country_stats else 1 %}
+    <canvas id="country-chart" height="100"></canvas>
-    {% for code, count in country_stats %}
-    <div style="display:flex;align-items:center;gap:.75rem;margin-bottom:.5rem">
-        <span style="width:5rem;text-align:right;font-size:.9rem;flex-shrink:0">{{ code|flag }} {{ code }}</span>
-        <div style="flex:1;background:var(--bg);border-radius:4px;height:1.5rem;overflow:hidden">
-            <div style="width:{{ (count / max_count * 100)|int }}%;height:100%;background:var(--primary);border-radius:4px;min-width:1.5rem;display:flex;align-items:center;justify-content:flex-end;padding-right:.4rem">
-                <span style="font-size:.75rem;font-weight:600;color:#fff">{{ count }}</span>
-            </div>
-        </div>
-    </div>
-    {% endfor %}
 </div>
+<script>
+new Chart(document.getElementById('country-chart'), {
+    type: 'bar',
+    data: {
+        labels: [{% for code, _ in country_stats %}'{{ code|flag }} {{ 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 %}
 
 <div class="dash-sections">