feat: port el-ui vessels — rename crates→vessels, add El source + manifests

ui/vessels/el-config/src/error.rs

@@ -0,0 +1,20 @@
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum ConfigError {
+    #[error("config key '{key}' not found")]
+    NotFound { key: String },
+
+    #[error("config key '{key}': expected {expected}, got '{got}'")]
+    TypeMismatch {
+        key: String,
+        expected: String,
+        got: String,
+    },
+
+    #[error("config parse error: {0}")]
+    ParseError(String),
+
+    #[error("config source error '{source_name}': {message}")]
+    SourceError { source_name: String, message: String },
+}