fix(api): Windows env hint + .env file loading fallback

When API key missing on Windows, hint about setx. Load .env from CWD
as fallback with simple key=value parser.

rust/crates/api/src/providers/anthropic.rs

@@ -732,7 +732,7 @@ fn now_unix_timestamp() -> u64 {
 fn read_env_non_empty(key: &str) -> Result<Option<String>, ApiError> {
     match std::env::var(key) {
         Ok(value) if !value.is_empty() => Ok(Some(value)),
-        Ok(_) | Err(std::env::VarError::NotPresent) => Ok(None),
+        Ok(_) | Err(std::env::VarError::NotPresent) => Ok(super::dotenv_value(key)),
         Err(error) => Err(ApiError::from(error)),
     }
 }