feat: replace direct P2P networking with iroh-gossip swarm and DHT peer discovery

Replace manually configured peer connections with a passphrase-based gossip
swarm. Peers sharing the same passphrase automatically discover each other
via mainline DHT and join an iroh-gossip topic.

Key changes:
- Add gossip.rs: iroh-gossip swarm with DHT bootstrap, heartbeat, and
  graceful shutdown that publishes neighbor list
- Add setup.rs: interactive config wizard replacing generate-config
- Add rate_limit.rs: token-bucket TX rate limiter derived from LoRa air time
- Remove network.rs: old direct P2P connection management
- Config: remove [[peers]] section, add passphrase to [bridge], update
  radio defaults to MeshCore US/Canada recommended values
- Packet: rename Envelope to GossipFrame, simplify wire format (drop
  version byte and seq number)
- Router: unify dedup into single content-hash cache, add rate limiting
- Identity: use ephemeral keypair per launch instead of persistent secret key
- Radio: improve config negotiation (GetConfig-first strategy), add device
  info tracking
- TUI: complete redesign with network/swarm panel, sparkline activity
  charts, color-coded RSSI/SNR, and shutdown overlay

Cargo.toml

@@ -9,6 +9,8 @@ description = "Peer-to-peer LoRa bridge using iroh and DongLoRa"
 [dependencies]
 donglora-client = { path = "../donglora/clients/rust" }
 iroh = "0.97"
+iroh-gossip = "0.97"
+mainline = { version = "6", features = ["async"] }
 tokio = { version = "1", features = ["full"] }
 clap = { version = "4", features = ["derive"] }
 serde = { version = "1", features = ["derive"] }
@@ -22,8 +24,10 @@ tracing-appender = "0.2"
 anyhow = "1"
 dirs = "6"
 bytes = "1"
+futures-lite = "2"
 rand = "0.9"
 hex = "0.4.3"
+sha2 = "0.10"
 tokio-util = { version = "0.7.18", features = ["rt"] }
 
 [lints.clippy]