feat: add Claude Usage Credits

[kurbezz]

Summary

Add Claude Usage Credits from the OAuth subscription usage response as a third quota row.

The row shows remaining credits for the monthly period using the Monthly: label.

Linked Issue

No issue exists. This is a focused enhancement that exposes the existing Claude OAuth extra_usage field.

OpenCode Validation

• Current production released OpenCode version tested: 1.18.25
• Why this version is relevant to the fix: it is the OpenCode version against which the Claude OAuth quota path and TUI/server rendering were tested.

Quality Checklist

• I ran pnpm run typecheck
• I ran pnpm run build
• I ran pnpm test
• This change is focused and avoids unrelated behavior changes
• I updated or added tests when behavior changed
• I updated docs when user-facing workflow, command, or config behavior changed — not applicable; no setup, configuration, command, or workflow changed
• For provider changes, I followed Provider Changes, or this does not apply — this modifies the existing Anthropic provider rather than adding a built-in provider