[codex] Make profile rank more prominent

[B-EtterDigital]

Summary

• Promote the achieved global rank from a small inline chip to a prominent profile-header element.
• Re-label the profile rank stat as Viberank position and keep the unranked state explicit.
• Add a small helper and regression test for rank display formatting.

Validation

• node test/profile-rank.test.mts passes: 6 tests.
• pnpm test passes: 19 existing ccusage tests and 6 profile rank tests.
• git diff --check and git diff --cached --check pass.
• GITHUB_ID=dummy GITHUB_SECRET=dummy NEXTAUTH_SECRET=dummy-secret NEXTAUTH_URL=http://localhost:3000 pnpm build compiles successfully, then fails prerendering /hire because Supabase server env vars are not configured locally.

Scope

Feature-only PR. It intentionally does not include PR #72 bug fixes, PR #73 expandable model list changes, or custom open-to-work email handling.

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