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/src/main.rs

@@ -11,6 +11,7 @@
 
 mod agent;
 mod app;
+mod stats;
 mod tray;
 mod ui;
 
@@ -28,14 +29,28 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         .build()?;
     let handle = runtime.handle().clone();
 
-    let options = eframe::NativeOptions {
+    #[cfg_attr(not(target_os = "macos"), expect(unused_mut))]
+    let mut options = eframe::NativeOptions {
+        // A fixed, small window; the whole layout targets this size.
         viewport: egui::ViewportBuilder::default()
             .with_title("tsunagi")
-            .with_inner_size([380.0, 540.0])
-            .with_min_inner_size([320.0, 360.0]),
+            .with_inner_size([360.0, 600.0])
+            .with_resizable(false)
+            // Start hidden: the app lives in the tray and the window opens from
+            // the menu.
+            .with_visible(false),
         ..Default::default()
     };
 
+    // macOS: live in the menu bar with no Dock icon (tray-only).
+    #[cfg(target_os = "macos")]
+    {
+        options.event_loop_builder = Some(Box::new(|builder| {
+            use winit::platform::macos::{ActivationPolicy, EventLoopBuilderExtMacOS};
+            builder.with_activation_policy(ActivationPolicy::Accessory);
+        }));
+    }
+
     eframe::run_native(
         "tsunagi",
         options,