feat: add configurable relay balance and model usage monitor

[MonoSeass]

Summary

Adds a configurable Codex++ user script for monitoring relay balance and per-model usage.

Features

• Displays the current relay balance in the Codex++ menu bar.
• Supports today, yesterday, and custom date-range queries.
• Shows per-model requests, input tokens, cache read tokens, cache write tokens, output tokens, total tokens, and actual cost.
• Calculates the effective actual billing multiplier as actual_cost / cost for the selected range.
• Supports automatic relay configuration from Codex++ and manual endpoint/API-key configuration.
• Keeps manually entered configuration in local storage; no API key is bundled in the script.

Compatibility and limitations

• Requires the Codex++ user-script environment; it is not a standalone browser/Tampermonkey script.
• Defaults to the sub2api-compatible /v1/usage endpoint.
• The usage response must provide balance or remaining, plus model_stats.
• The actual multiplier is a weighted result for the selected query range, not a provider-wide pricing setting.
• The upstream relay must expose the required balance and usage data.

Validation

• node --check scripts/codex-relay-balance.js
• Parsed index.json successfully.
• Verified the registered SHA-256 hash matches the script:
  8196E88932C5631B92A9E3CA9CCD569465D4FD8668E34F3B7655F6095ED3D592
• git diff --check passed.
• Verified in the Codex++ environment with the configured relay.

Update in 0.3.3

• Non-2xx responses now include the upstream error detail and requested path when available, so HTTP 500 responses can be diagnosed instead of showing only a generic status.