fix: standalone MITM — remove HTTPS_PROXY with iptables, fix is_agent detection

- Only set HTTPS_PROXY/HTTP_PROXY when iptables UID isolation is NOT
  available. With iptables, double-proxying caused profile picture
  fetches to fail with 'lookup http' DNS errors.
- Fix is_agent detection: handle JSON with spaces after colons
  ("requestType": "agent" vs "requestType":"agent")
- Suppress wrapper-not-installed warning in standalone mode
- Show 'iptables (standalone)' in banner instead of 'not installed'

src/mitm/proxy.rs

@@ -551,9 +551,9 @@ async fn handle_http_over_tls(
                 let raw_body = super::modify::dechunk(body_slice);
 
                 // Only modify "agent" requests, not "checkpoint" (LS internal)
-                let is_agent = raw_body
-                    .windows(20)
-                    .any(|w| w == b"\"requestType\":\"agent" || w == b"requestType\":\"agent\"");
+                let body_str = String::from_utf8_lossy(&raw_body);
+                let is_agent = body_str.contains("\"requestType\":\"agent\"")
+                    || body_str.contains("\"requestType\": \"agent\"");
 
                 if is_agent {
                     if let Some(modified_body) = super::modify::modify_request(&raw_body) {