fix(#505,#506): bump firmware to v0.6.4 + no-signal UI indicator

Fixes #505 — version string mismatch:
- firmware/esp32-csi-node/version.txt: 0.6.2 → 0.6.4
- .github/workflows/firmware-ci.yml: add verify-embedded-version step
  that extracts the esp_app_desc version string from the built binary and
  fails CI if it doesn't match version.txt. Prevents the v0.6.3 regression
  (tagged without bumping version.txt) from recurring.

Fixes #506 — stale skeleton shown after ESP32 unplugged:
- csi_pipeline.rs: add CsiPipelineState::last_csi_received (Instant),
  stamped on every process_frame() call; PipelineOutput gains csi_live:bool
  (true iff a real frame arrived within the last 5 s).
- stream.rs: /api/splats now includes csi_live in the JSON response.
- viewer.html: add #no-signal banner (red, centered overlay) shown when
  the server reports csi_live=false in live/remote transport mode.
  Skeleton is cleared while the banner is visible so stale pose data
  is not presented as live activity.

Co-Authored-By: claude-flow <ruv@ruv.net>

firmware/esp32-csi-node/version.txt

@@ -1 +1 @@
-0.6.2
+0.6.4