[refactor](trx-server): consolidate protocol logic with trx-protocol

Use centralized trx-protocol crate for:
- parse_mode and mode string parsing
- parse_envelope with fallback behavior
- command mapping (ClientCommand -> RigCommand)
- token validation with SimpleTokenValidator

Removes 116 lines of duplicate code. Wraps validator in Arc for
safe sharing across async tasks. No behavior changes, all tests pass.

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

src/trx-server/Cargo.toml

@@ -24,3 +24,4 @@ opus = "0.3"
 trx-backend = { path = "trx-backend" }
 trx-core = { path = "../trx-core" }
 trx-ft8 = { path = "../trx-ft8" }
+trx-protocol = { path = "../trx-protocol" }