feat(usage): track cloud quotas and expose :limits

Acceptance Criteria:\n- header shows hourly/weekly usage with colored thresholds\n- :limits command prints persisted usage data and quotas\n- token usage survives restarts and emits 80%/95% toasts

Test Notes:\n- cargo test -p owlen-core usage

docs/configuration.md

@@ -177,10 +177,14 @@ base_url = "http://localhost:11434"
 enabled = true
 base_url = "https://ollama.com"
 api_key_env = "OLLAMA_API_KEY"
+hourly_quota_tokens = 50000
+weekly_quota_tokens = 250000
 ```
 
 Requests target the same `/api/chat` endpoint documented by Ollama and automatically include the API key using a `Bearer` authorization header. If you prefer not to store the key in the config file, either rely on `api_key_env` or export the `OLLAMA_API_KEY` environment variable manually (legacy names `OLLAMA_CLOUD_API_KEY` and `OWLEN_OLLAMA_CLOUD_API_KEY` continue to work, but Owlen will emit a warning). Owlen normalises the base URL automatically—it enforces HTTPS, trims trailing slashes, and accepts both `https://ollama.com` and `https://api.ollama.com` without rewriting the host.
 
+The quota fields are optional and purely informational—they are never sent to the provider. Owlen uses them to display hourly/weekly token usage in the chat header, emit pre-limit toasts at 80 % and 95 %, and power the `:limits` command. Adjust the numbers to reflect the soft limits on your account or remove the keys altogether if you do not want usage tracking.
+
 > **Tip:** If the official `ollama signin` flow fails on Linux v0.12.3, follow the [Linux Ollama sign-in workaround](#linux-ollama-sign-in-workaround-v0123) in the troubleshooting guide to copy keys from a working machine or register them manually.
 
 ### Managing cloud credentials via CLI