Fix leaderboard savings lookup after provider-key rename (#634 follow-up)

[jonsaadfalcon]

What

Follow-up fix for a regression introduced by #634.

That PR renamed the Anthropic cost-comparison provider key claude-opus-4.6 → claude-fable-5 across the backend and most frontend consumers, but missed one: App.tsx computes the leaderboard dollar_savings figure by finding the Anthropic entry by key:

const claudeEntry = data.per_provider.find((p) => p.provider === 'claude-opus-4.6');
const dollarSavings = claudeEntry ? claudeEntry.total_cost : 0;

After the rename that .find() returns undefined, so this path silently submitted dollar_savings = 0 to the leaderboard. This one-line fix points the lookup at the new key.

How it was found

Post-merge exhaustive grep for consumers of the renamed keys (grep '.provider ===' / 'providerMap[' / per_provider.find). This was the only remaining stale consumer — all other lookups (dashboard providerMap, PROVIDER_COLORS) already use the new keys.

Verification

• tsc --noEmit — clean
• Confirmed no other claude-opus-4.6 / gpt-5.3 string-literal key references remain repo-wide (the claude-opus-4-6 dashed id in CommandPalette.tsx is the real model-selector entry, intentionally unchanged).

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