Tooltip formatter の TypeScript 型エラーを修正

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/app/weather/page.tsx

@@ -214,7 +214,10 @@ export default function WeatherPage() {
                       domain={['auto', 'auto']}
                     />
                     <Tooltip
-                      formatter={(value: number | null) => value == null ? '—' : `${value.toFixed(1)}℃`}
+                      formatter={(value: unknown) => {
+                        if (value == null || typeof value !== 'number') return '—';
+                        return `${value.toFixed(1)}℃`;
+                      }}
                     />
                     <Legend />
                     <Line
@@ -266,8 +269,8 @@ export default function WeatherPage() {
                       tickFormatter={(v) => `${v}h`}
                     />
                     <Tooltip
-                      formatter={(value: number | null, name: string) => {
-                        if (value == null) return '—';
+                      formatter={(value: unknown, name: unknown) => {
+                        if (value == null || typeof value !== 'number') return '—';
                         return name === '降水量' ? `${value} mm` : `${value} h`;
                       }}
                     />