Fix touch crash + make rain a reachability signal + calm the alarm churn

TOUCH CRASH (the blue screen): app_on_touch ran a blocking WebSocket
send (up to 5s) directly in the LVGL touch callback, stalling the
MIPI-DSI flush into a garbage/blue frame + task-watchdog reboot on
every tap. Now touch_cb only gives a semaphore; a dedicated app_task
does the blocking sends, audio, and face changes off the render
thread. WS send timeout cut 5s to 1.5s as belt-and-braces.

RAIN = REACHABILITY (user request): rain now falls only while the
gateway WebSocket is live (gated on gw_connected in rain_tick). It
drains gracefully on disconnect, resumes on reconnect, a genuine
glanceable reachable signal. Idle density bumped 2 to 4 so
connected-idle reads distinctly from disconnected-black.

CALM THE WEDGE CHURN (user request): transient wifi/WS drops no longer
slam to the x_x error face or a CONNECTING banner. Boot goes straight
to the calm idle face (dry until connected). Only a sustained 30s+
outage escalates to x_x (clock_cb); the ~15s wedge-recovery just shows
a brief rain pause.

Two fixes from adversarial concurrency review before flashing:
- persistent single capture task (was xTaskCreate per utterance; a
  rapid re-tap or WS-stop-vs-app-start race could put two readers on
  one mic/I2S handle and corrupt the codec)
- reset s_talking on disconnect (app_on_disconnect) so the first tap
  after a mid-utterance drop starts fresh, not the stop branch

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

firmware/main/net.c

@@ -76,7 +76,7 @@ static void wifi_event(void *arg, esp_event_base_t base, int32_t id, void *data)
     } else if (base == WIFI_EVENT && id == WIFI_EVENT_STA_DISCONNECTED) {
         wifi_event_sta_disconnected_t *dc = (wifi_event_sta_disconnected_t *)data;
         ESP_LOGW(TAG, "wifi disconnected (reason %d), retrying", dc ? dc->reason : -1);
-        face_set(FACE_ERROR);
+        /* rain drains via the gw_connected() gate; face stays calm (see face.c) */
         vTaskDelay(pdMS_TO_TICKS(3000));
         esp_wifi_connect();
     } else if (base == IP_EVENT && id == IP_EVENT_STA_GOT_IP) {