feat: add credential store settings panel

[aniemerg]

Summary

• add /api/me/credential-store to aggregate per-user provider and connection credential metadata
• expose iron-control namespace, principal, broker, and static-secret references without returning token material
• add a Settings > Credential Store power panel while leaving the existing beginner Codex/Claude/GitHub controls intact

Validation

• git diff --check
• Not run: pnpm lint / typecheck / tests because this pod could not fetch pnpm@10.33.0 via Corepack and no node_modules are present. Corepack failed fetching https://registry.npmjs.org/pnpm/-/pnpm-10.33.0.tgz.

Prompted by: Allan Niemerg (@allan)

[github-actions]

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