[refactor](trx-rs): remove shared-library plugin system

Drop the plugin loading infrastructure (libloading-based dynamic .so/.dylib/.dll
loading) from both trx-server and trx-client. The feature was unused and posed an
unnecessary security risk by executing arbitrary native code from disk.

Removed:
- src/trx-app/src/plugins.rs (plugin discovery, validation, FFI registration)
- examples/trx-plugin-example/ (cdylib example plugin)
- libloading dependency from trx-app
- load_backend_plugins / load_frontend_plugins calls from server and client
- Plugin documentation from README.md and CLAUDE.md

https://claude.ai/code/session_01DTEUpz3XPUeWmz74NeaFgb
Signed-off-by: Claude <noreply@anthropic.com>

src/trx-client/src/main.rs

@@ -10,7 +10,6 @@ mod remote_client;
 use std::collections::HashMap;
 use std::net::{IpAddr, SocketAddr};
 use std::path::PathBuf;
-use std::ptr::NonNull;
 use std::time::Duration;
 
 use bytes::Bytes;
@@ -20,7 +19,7 @@ use tokio::sync::{broadcast, mpsc, watch};
 use tokio::task::JoinHandle;
 use tracing::{error, info};
 
-use trx_app::{init_logging, load_frontend_plugins, normalize_name};
+use trx_app::{init_logging, normalize_name};
 use trx_core::audio::AudioStreamInfo;
 
 use trx_core::decode::DecodedMessage;
@@ -147,9 +146,6 @@ async fn async_init() -> DynResult<AppState> {
 
     init_logging(cfg.general.log_level.as_deref());
 
-    let frontend_ctx_ptr = NonNull::from(&mut frontend_reg_ctx).cast();
-    let _plugin_libs = load_frontend_plugins(frontend_ctx_ptr);
-
     if let Some(ref path) = config_path {
         info!("Loaded configuration from {}", path.display());
     }