Respect XDG_DATA_HOME when syncing auth.json

[exiast]

Problem

On Linux setups where OpenCode uses a custom XDG_DATA_HOME, this plugin syncs Anthropic OAuth creds to ~/.local/share/opencode/auth.json instead of the active OpenCode data dir.

Repro

1. Set XDG_DATA_HOME to a non-default path (example: /home/user/work/.local/share).
2. Use opencode-claude-auth@latest with valid Claude credentials (~/.claude/.credentials.json).
3. Run OpenCode and check providers.

Observed

• Plugin debug log shows writes to ~/.local/share/opencode/auth.json.
• OpenCode reads credentials from $XDG_DATA_HOME/opencode/auth.json.
• Anthropic provider/models are missing until manual copy is done.

Expected

Plugin should write to $XDG_DATA_HOME/opencode/auth.json on non-Windows (with fallback to ~/.local/share/opencode/auth.json when XDG_DATA_HOME is unset).

Notes

This also affects persisted account source path (claude-account-source.txt) for the same reason.

I have a fix ready and can open PR.