fix(sensing-server): WS broadcast emits effective_source() not hardcoded "esp32" (closes #618) (#621)

Reported by @ArnonEnbar with a complete reproduction.

broadcast_tick_task() re-emits the cached `latest_update` every tick so
pose WS clients keep getting data even when ESP32 pauses between
frames. The `source` field of that cached update was set to "esp32" at
the moment a fresh ESP32 frame was last decoded (main.rs:3885, :4136).

After the ESP32 loses power or network, no fresh frame is decoded —
the cached `latest_update` is still re-broadcast every tick with the
stale source: "esp32" baked in. UI's "Sensing" tab keeps showing
"LIVE — ESP32 HARDWARE Connected" with frozen vitals/features/
classification re-broadcast indefinitely. REST `/health` correctly
reports source: "esp32:offline" (via effective_source(), which checks
last_esp32_frame elapsed time against ESP32_OFFLINE_TIMEOUT=5s) — but
the WS broadcast path was the one consumer that didn't call it.

Fix: clone the cached update per tick, overwrite source with
s.effective_source(), then serialize and broadcast. UI now switches to
"esp32:offline" on the same 5s budget as the REST surface.

cargo build -p wifi-densepose-sensing-server --no-default-features:
17s, no errors (1 pre-existing unused-import warning unchanged).

v2/crates/wifi-densepose-sensing-server/src/main.rs

@@ -4331,7 +4331,18 @@ async fn broadcast_tick_task(state: SharedState, tick_ms: u64) {
             if s.tx.receiver_count() > 0 {
                 // Re-broadcast the latest sensing_update so pose WS clients
                 // always get data even when ESP32 pauses between frames.
-                if let Ok(json) = serde_json::to_string(update) {
+                //
+                // Issue #618: overwrite `source` with `effective_source()`
+                // before each broadcast so a stale latest_update (frozen
+                // payload from a now-offline ESP32) is emitted with
+                // `source: "esp32:offline"` instead of `source: "esp32"`.
+                // The REST `/health` endpoint already does this; before
+                // this fix the WS path was the only consumer that didn't,
+                // so the UI's "LIVE — ESP32 HARDWARE Connected" banner
+                // stayed green long after the hardware went away.
+                let mut tagged = update.clone();
+                tagged.source = s.effective_source();
+                if let Ok(json) = serde_json::to_string(&tagged) {
                     let _ = s.tx.send(json);
                 }
             }