feat: add OpenClaw User-Agent toggle, default off

Add a switch in the OpenClaw provider form to optionally send a browser
User-Agent header. The toggle defaults to off — only providers that
explicitly include headers in their preset or config will have it enabled.

Remove the previous auto-injection logic that force-added User-Agent on
every preset load and new provider creation.

src-tauri/src/openclaw_config.rs

@@ -100,6 +100,8 @@ pub struct OpenClawProviderConfig {
     pub api: Option<String>,
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub models: Vec<OpenClawModelEntry>,
+    #[serde(default, skip_serializing_if = "HashMap::is_empty")]
+    pub headers: HashMap<String, String>,
     #[serde(flatten)]
     pub extra: HashMap<String, Value>,
 }