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-server/trx-backend/Cargo.toml

@@ -0,0 +1,20 @@
+# SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+[package]
+name = "trx-backend"
+version = "0.1.0"
+edition = "2021"
+
+[features]
+default = ["ft817"]
+ft817 = ["dep:trx-backend-ft817"]
+
+[dependencies]
+trx-core = { path = "../../trx-core" }
+trx-backend-ft817 = { path = "trx-backend-ft817", optional = true }
+tokio = { workspace = true, features = ["full"] }
+tokio-serial = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+tracing = { workspace = true }