refactor: prefix private data members with m_

Rename all trailing-underscore private data members to the m_ prefix
required by AGENTS.md, across the auth, crypto, gip, mt76, and usb
transport classes plus the app session classes. Pure rename; no
behavior change. Full build and test suite pass.

Co-Authored-By: qwen3.8-27b@q3_k_xl: renamed members to m_ prefix

include/usb/usb_transport.hpp

@@ -125,7 +125,7 @@ private:
     static void on_read_completion(void *refcon, std::int32_t result, void *arg0);
     static void on_dongle_terminated(void *refcon, std::uint32_t iter);
 
-    std::unique_ptr<state> state_;
+    std::unique_ptr<state> m_state;
 };
 
 // TRUE if an Xbox Wireless Dongle is connected (any supported PID).