[refactor](trx-client): remove Qt/QML frontend support

Remove the Linux-only Qt/QML frontend (trx-frontend-qt) crate and all
references to it from the workspace, trx-client binary, configuration,
and documentation. This prepares for replacement with a native macOS
AppKit frontend.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

trx-client.toml.example

@@ -47,7 +47,3 @@ listen = "127.0.0.1"
 port = 0
 # List of accepted bearer tokens (empty = no auth)
 # auth.tokens = ["example-token"]
-
-[frontends.qt]
-# Enable Qt/QML GUI frontend (Linux only, requires system Qt6)
-enabled = false