add(deepseek_usage): DeepSeek API credit & balance monitor (#220)
* add(deepseek_usage): add DeepSeek usage balance monitor plugin
* add(deepseek_usage): add thumbnail.webp
* chore(deepseek_usage): update plugin id to coder/deepseek_usage
* fix(deepseek_usage): update 960x540 thumbnail, README IPC docs, and plugin.toml schema
* fix(deepseek_usage): fix panel ID, normalize ui.graph values 0-1, declare xdg-open dependency
* fix(deepseek_usage): replace thumbnail with official Noctalia generator asset
