Tray UI: start hidden, fixed window, grid layout, traffic + peers

Reworks the tray window per feedback and fills it in.

- Starts hidden in the tray (no window on launch); the window opens from the
  menu. On macOS the app now uses the Accessory activation policy, so it lives
  in the menu bar with no Dock icon (via a winit event-loop hook, pinned to
  eframe's winit). Fixed, non-resizable 360x600 window; the layout targets it.
- Tidied layout: top bar (title + refresh), a bottom panel with the join form
  and the action banner, and a scrolling list of network tiles between them.
  Labels and values sit on two-column grids so they align; the join name and
  secret inputs are the same fixed width.
- A ⟳ button beside the secret generates a fresh random secret
  (NetworkSecret::generate), for creating a network.
- Each network tile shows id, range, this device's address, peers
  (connected/known), the active and broadcast switches, a Leave button, and a
  collapsing "peers (N)" section with a formatted per-peer table (short key,
  address, handshake age, tx/rx packets, rate).
- Traffic: a new sampler turns the agent's cumulative packet counters into a
  packets-per-second rate per network and per peer, from the deltas between
  snapshots (the snapshot now carries a generation and timestamp so each
  refresh is sampled once). The network tile shows the current rate and a small
  hand-drawn sparkline of recent rates (no extra dependency).

Note: rates are packets/s — the report has no byte counters yet; bytes would
need an agent change. Builds and is clippy-clean (-D warnings) on macOS; the UI
has not been run in this headless session.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/tsunagi-tray/Cargo.toml

@@ -25,5 +25,10 @@ tokio = { version = "1.53", features = ["rt", "rt-multi-thread", "sync", "time",
 tracing.workspace = true
 tracing-subscriber.workspace = true
 
+# macOS only: set the app's activation policy to Accessory so it lives in the
+# menu bar with no Dock icon. Pinned to eframe's winit so the types unify.
+[target.'cfg(target_os = "macos")'.dependencies]
+winit = "0.30"
+
 [lints]
 workspace = true