fix: check response.ok before parsing JSON in fetch calls

Fix JSON parsing errors when server returns non-JSON responses:
- Check response.ok before calling response.json()
- Add fallback error messages using status text
- Apply fix to all fetch calls across 12 components

This prevents 'JSON.parse: unexpected character' errors when
server returns HTML error pages or other non-JSON responses.

src/components/RecalculateEloButton.tsx

@@ -18,6 +18,17 @@ export function RecalculateEloButton() {
           'Content-Type': 'application/json',
         },
       })
+      
+      if (!response.ok) {
+        try {
+          const errorData = await response.json()
+          alert(`Error: ${errorData.error || 'Failed to recalculate'}`)
+        } catch {
+          alert(`Error: ${response.status} ${response.statusText}`)
+        }
+        return
+      }
+      
       const data = await response.json()
       
       if (data.success) {