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>

.github/workflows/firmware-ci.yml

@@ -98,6 +98,32 @@ jobs:
             echo "Flash image integrity verified"
           fi
 
+      - name: Verify embedded version string matches version.txt (fixes #505)
+        working-directory: firmware/esp32-csi-node
+        run: |
+          EXPECTED=$(cat version.txt | tr -d '[:space:]')
+          BIN=build/esp32-csi-node.bin
+          # Extract version from ESP-IDF app_desc: magic 0xABCD5432 at offset 0
+          # followed by version string at offset 16, null-terminated, max 32 chars.
+          EMBEDDED=$(python3 -c "
+import struct, sys
+data = open('$BIN','rb').read()
+magic = struct.pack('<I', 0xABCD5432)
+i = data.find(magic)
+if i < 0:
+    sys.exit('app_desc magic not found')
+ver = data[i+16:i+48].split(b'\\x00',1)[0].decode('ascii','replace')
+print(ver)
+" 2>&1)
+          echo "Expected version: $EXPECTED"
+          echo "Embedded version: $EMBEDDED"
+          if [ "$EMBEDDED" != "$EXPECTED" ]; then
+            echo "::error::Version string mismatch! version.txt='$EXPECTED' but binary reports '$EMBEDDED'."
+            echo "::error::Ensure version.txt is updated before building and tagging."
+            exit 1
+          fi
+          echo "Version string verified: $EMBEDDED"
+
       - name: Stage release binaries with variant-specific names
         working-directory: firmware/esp32-csi-node
         run: |