[feat](trx-frontend): add context types for explicit initialization

Create explicit context types for frontend registration and runtime:

FrontendRegistrationContext:
- register_frontend(name, spawner) - register a frontend
- is_frontend_registered(name) - check if registered
- registered_frontends() -> Vec<String> - list all frontends
- spawn_frontend(name, ...) -> DynResult - spawn a frontend

FrontendRuntimeContext (NEW):
- audio_rx: broadcast channel for audio RX
- audio_tx: mpsc channel for audio TX
- audio_info: watch channel for audio stream metadata
- decode_rx: broadcast channel for decoded messages
- aprs_history: Arc<Mutex<VecDeque>> for APRS decode history
- cw_history: Arc<Mutex<VecDeque>> for CW decode history
- ft8_history: Arc<Mutex<VecDeque>> for FT8 decode history
- auth_tokens: HashSet for authentication

Replaces global mutable state with explicit context that can be
threaded through bootstrap. Maintains global API for compatibility.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-client/trx-frontend/Cargo.toml

@@ -8,5 +8,6 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+bytes = "1"
 trx-core = { path = "../../trx-core" }
 tokio = { workspace = true, features = ["sync"] }