feat: real-time packet arcs, traceroute tracer, and live map polish

[SimmerV]

Builds on the existing SSE stream to animate the map from real events. Adds one
WebGL layer (ActivityLayer) plus the wiring to drive it.

Packet arcs

• Animates each packet event as a comet from from (transmitter) to each
  sender (gateway that heard it).
• Dedupes multi-gateway uplinks by mesh id into one origin pulse + one arc per
  gateway.
• Colored by packet type, weighted by RSSI/SNR. Position-less origin renders a
  gateway ripple instead of a fake line.
• Snaps endpoints to the containing cluster when clustering is on.
• On by default; opt-out via Settings → Animations.

Traceroute tracer

• mqtt.py handle_traceroute now publishes a traceroute SSE event (+ test).
• Frontend animates a comet along the real [from, …route, to] path, skipping
  position-less hops, deduped to the most complete path per mesh id.

Map liveness

• Cluster donut ratio tweens between real online/total values.
• Live cluster hover card: membership, ratio, role breakdown, last-heard list.
• Node brightness encodes last-heard recency; new nodes appear without reload.
• Node activity sparkline in the details panel (15-min, history-seeded + live).
• Legend updated: live-packet type-color key (shared with the arcs), recency,
  traceroute.

Performance

• ActivityLayer: persistent ring buffer, time-in-shader animation, self-
  stopping rAF, vsync-aligned pacing, batched per-flush uploads,
  bounded draw range, 30fps cap.
• Cluster donut re-drapes terrain only on camera movement.
• Per-flush arc cap + mesh-id dedup for backpressure.
• Comet duration scales with on-screen distance (constant visual speed).
• Respects prefers-reduced-motion.