add efficiency leaderboard metric

[kevinfosterNG]

What changed

• add an Efficiency tab to the homepage leaderboard beside Cost and Tokens
• compute efficiency client-side as totalTokens / totalCost and display it as a Tok/$ column
• fetch and sort the full filtered result set client-side for efficiency mode so rankings stay accurate
• update homepage and README copy to mention the new efficiency metric

Why

The homepage currently highlights spend and token volume, but it does not surface which submissions get the most tokens per dollar. This adds a focused third ranking mode without changing the database schema.

Impact

Users can now switch the root leaderboard to an efficiency-oriented view and identify the most cost-efficient submissions, while existing cost and token rankings remain unchanged.

Validation

• pnpm build
• pnpm exec tsc --noEmit
  Fails in the current repo due to pre-existing type/config issues unrelated to this change.
• pnpm lint
  Fails in the current repo because the script still uses next lint, which is invalid under the current Next.js setup.
• pnpm test
  Fails in the current repo because the script runs node directly on test/ccusage.test.mts.

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