feat: overhaul radio reconnection, TUI layout, DHT strategy, and signal handling

Radio reconnection:
- Add connect_mux_auto() to donglora-client — mux-only connect that never
  silently falls back to USB serial (matches Python client's sticky mux)
- Fix codec.rs read_frame() to handle WouldBlock alongside TimedOut
- Add liveness ping (2s) to detect dead mux connections
- Treat transmit errors as fatal (trigger reconnect)
- Faster backoff: 250ms initial, 5s max, resets after stable connection

TUI improvements:
- Redesign packet log: RF+Net columns show full packet path (RX→TX),
  hash column with dedup ellipsis, fixed-width columns throughout
- Move activity charts to right column (fills remaining width)
- 1-second sparkline buckets instead of 10-second
- Radio panel shows "(disconnected)" with red border when mux is down
- Header: status pills left, clock with TZ right-justified, Ctrl-C quits
- Local time via jiff crate with proper timezone abbreviation

DHT strategy:
- Alone: read every 3s, merge-publish every 30s (preserves existing peers)
- Connected: publish authoritative [self + neighbors], purging stale peers
- Publish immediately on first neighbor join

Signal handling:
- SIGINT and SIGTERM both trigger graceful shutdown via cancellation token
- All paths go through swarm.shutdown() to update DHT before exit

Cargo.lock

@@ -862,6 +862,7 @@ dependencies = [
  "hex",
  "iroh",
  "iroh-gossip",
+ "jiff",
  "mainline",
  "rand",
  "ratatui",
@@ -1984,6 +1985,47 @@ version = "1.0.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8f42a60cbdf9a97f5d2305f08a87dc4e09308d1276d28c869c684d7777685682"
 
+[[package]]
+name = "jiff"
+version = "0.2.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1a3546dc96b6d42c5f24902af9e2538e82e39ad350b0c766eb3fbf2d8f3d8359"
+dependencies = [
+ "jiff-static",
+ "jiff-tzdb-platform",
+ "log",
+ "portable-atomic",
+ "portable-atomic-util",
+ "serde_core",
+ "windows-sys 0.61.2",
+]
+
+[[package]]
+name = "jiff-static"
+version = "0.2.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2a8c8b344124222efd714b73bb41f8b5120b27a7cc1c75593a6ff768d9d05aa4"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "jiff-tzdb"
+version = "0.1.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c900ef84826f1338a557697dc8fc601df9ca9af4ac137c7fb61d4c6f2dfd3076"
+
+[[package]]
+name = "jiff-tzdb-platform"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "875a5a69ac2bab1a891711cf5eccbec1ce0341ea805560dcd90b7a2e925132e8"
+dependencies = [
+ "jiff-tzdb",
+]
+
 [[package]]
 name = "js-sys"
 version = "0.3.94"
@@ -2748,6 +2790,15 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "portable-atomic-util"
+version = "0.2.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "091397be61a01d4be58e7841595bd4bfedb15f1cd54977d79b8271e94ed799a3"
+dependencies = [
+ "portable-atomic",
+]
+
 [[package]]
 name = "portmapper"
 version = "0.15.0"