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/Cargo.toml

@@ -17,5 +17,5 @@ tracing-subscriber = { workspace = true }
 clap = { workspace = true, features = ["derive"] }
 dirs = "6"
 libloading = "0.8"
-trx-backend = { path = "../trx-backend" }
+trx-backend = { path = "trx-backend" }
 trx-core = { path = "../trx-core" }