Fix leaderboard savings lookup after provider-key rename (#635)

PR #634 renamed the Anthropic cost-comparison provider key
`claude-opus-4.6` -> `claude-fable-5` but missed one consumer:
App.tsx looks up the Anthropic entry by that key to compute the
`dollar_savings` value submitted to the leaderboard. After the rename
`per_provider.find(p => p.provider === 'claude-opus-4.6')` returned
undefined, so this path silently submitted dollar_savings = 0.

Point the lookup at the new key.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -89,7 +89,7 @@ export default function App() {
           setSavings(data);
           if (optInEnabled && optInDisplayName && data) {
             const claudeEntry = data.per_provider.find(
-              (p) => p.provider === 'claude-opus-4.6',
+              (p) => p.provider === 'claude-fable-5',
             );
             const dollarSavings = claudeEntry ? claudeEntry.total_cost : 0;
             const energySaved = data.per_provider.reduce(