refactor: nest trx-frontend under trx-client, trx-backend under trx-server

Move the frontend and backend crate trees to live physically under their
respective binary crate directories, grouping related code together
without merging crate boundaries. Also flatten sub-crate nesting by
moving them out of src/ subdirectories into direct children.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/trx-client/trx-frontend/Cargo.toml

@@ -0,0 +1,12 @@
+# SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+[package]
+name = "trx-frontend"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+trx-core = { path = "../../trx-core" }
+tokio = { workspace = true, features = ["sync"] }