[client] clipboard: add transport provider interface

Route clipboard exchange through a provider interface. Connected
transports take priority while SPICE remains the fallback.

Track asynchronous X11 and Wayland transfers by request, preserving
clipboard ownership across provider changes.

Use PureSpice clipboard status notifications for capability and
lifecycle updates.

client/src/main.h

@@ -123,13 +123,6 @@ struct AppState
   LG_Lock              lgrLock;
   bool                 useDMA;
 
-  bool                 cbAvailable;
-  _Atomic(PSDataType)  cbRemoteType;
-  PSDataType           cbWriteType;
-  bool                 cbChunked;
-  size_t               cbXfer;
-  struct ll          * cbRequestList;
-
   LG_Transport              * transport;
   const LG_TransportOps     * transportOps;
   LG_TransportFeatureFlags    transportFeatures;
@@ -243,13 +236,6 @@ struct AppParams
   bool                 audioSyncVolume;
 };
 
-struct CBRequest
-{
-  PSDataType          type;
-  LG_ClipboardReplyFn replyFn;
-  void              * opaque;
-};
-
 struct KeybindHandle
 {
   int          sc;