docs(readme): fix contradictory just dev command description

The Common dev commands section described just dev as running only the
desktop app, but it actually starts both the relay and the desktop app
together. Corrected the description to match the Quick start section and
the justfile recipe. Also added just desktop-dev to the command list,
since it is the command that runs only the desktop frontend.

Closes #5160

Co-authored-by: Brad Groux <bradgroux@users.noreply.github.com>
Signed-off-by: Brad Groux <bradgroux@users.noreply.github.com>
Signed-off-by: Brad Groux <bradgroux@hotmail.com>
Signed-off-by: Brad Groux <3053586+BradGroux@users.noreply.github.com>

Author: BradGroux

README.md

@@ -259,7 +259,8 @@ All defaults work out of the box. Override via `.env`. Full reference in [`.env.
 ```bash
 just setup          # Docker, migrations, desktop deps
 just relay          # Run the relay
-just dev            # Run the desktop app
+just dev            # Run the relay + desktop app together
+just desktop-dev    # Run the desktop app frontend only (connects to an existing relay)
 just build          # Build the Rust workspace
 just check          # fmt + clippy + desktop check
 just test-unit      # Unit tests (no infra required)