[refactor](trx-config): extract client/server config into a shared crate

The setup wizard, the server and the client each carried their own idea of
what a valid config looks like: trx-configurator validated with hand-written
toml_edit key lists while the binaries validated with serde plus their own
validate().  Nothing kept the three in sync.

Move ServerConfig, ClientConfig, the section loader, the shared validators and
the endpoint-URL parsing into a new trx-config crate that all three depend on,
so there is one definition of the config to drift from.  The binaries keep a
thin crate::config re-export so their internal paths are unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyX26FCpMQxiBoC7r5K1A7
Signed-off-by: Stan Grams <sjg@haxx.space>

Cargo.toml

@@ -18,6 +18,7 @@ members = [
     "src/trx-core",
     "src/trx-protocol",
     "src/trx-app",
+    "src/trx-config",
     "src/trx-reporting",
     "src/trx-server",
     "src/trx-server/trx-backend",