Add Z.AI Coding Plan usage provider with OpenCode key discovery

[Phototonic]

Summary

• Add a managed Z.AI Coding Plan provider with manual API-key setup and one-time OpenCode prefill (zai-coding-plan, then zai).
• Store accepted keys only in YapCap-managed account storage; detect usable OpenCode API entries without retaining credential data.
• Fetch fixed-origin quota data with bounded Bearer-to-raw authorization compatibility retry, redirect blocking, conservative error handling, and deterministic 5 Hour / Weekly / MCP display.
• Add account lifecycle, localization, provider assets, synthetic demo data, sanitized fixtures, specification, and README coverage.

Validation

• cargo fmt --all
• just check
• cargo test --all (772 passed)
• cargo fmt --all -- --check
• git diff --check
• Targeted security/lifecycle review
• Manual local validation: OpenCode prefill created a Z.AI account and live 5-hour/weekly quotas matched the expected amounts.

Scope

• Supports global personal Coding Plan accounts only; regional/team/promotional balances, OAuth, OpenCode writes, and alternate endpoints are intentionally excluded.