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/desklock_main.c

@@ -2,10 +2,18 @@
  *
  * Boot: display + face -> audio (gong) -> Wi-Fi (C6/ESP-Hosted) -> gateway WS.
  * Interaction (phase 2): touch-to-talk — tap to speak, tap again to finish.
+ *
+ * Touch is DECOUPLED from the LVGL thread: the touch callback only signals a
+ * semaphore; all the real work (blocking WebSocket sends, audio, face changes)
+ * runs in app_task. A blocking WS send on the LVGL render thread stalls the
+ * MIPI-DSI flush -> blue/garbage frame + task-watchdog reboot.
  */
 
 #include "esp_log.h"
 #include "esp_system.h"
+#include "freertos/FreeRTOS.h"
+#include "freertos/semphr.h"
+#include "freertos/task.h"
 #include "bsp/esp-bsp.h"
 
 #include "desklock.h"
@@ -15,13 +23,31 @@
 
 static const char *TAG = "desklock";
 
+static SemaphoreHandle_t s_touch_sem;
 static bool s_talking;
 
+/* Called from the LVGL touch callback — MUST be fast and non-blocking. */
 void app_on_touch(void)
+{
+    if (s_touch_sem != NULL) {
+        xSemaphoreGive(s_touch_sem);
+    }
+}
+
+/* Called from the WS event task when the link drops: forget any half-utterance
+ * so the next tap starts a fresh one instead of hitting the "stop" branch. */
+void app_on_disconnect(void)
+{
+    s_talking = false;
+}
+
+/* Runs in app_task (normal context): blocking sends here don't stall rendering. */
+static void handle_touch(void)
 {
     face_activity();
 
     if (!gw_connected()) {
+        s_talking = false; /* link gone; nothing to talk to */
         return;
     }
     if (!s_talking) {
@@ -42,17 +68,29 @@ void app_on_touch(void)
     }
 }
 
+static void app_task(void *arg)
+{
+    (void)arg;
+    for (;;) {
+        if (xSemaphoreTake(s_touch_sem, portMAX_DELAY) == pdTRUE) {
+            vTaskDelay(pdMS_TO_TICKS(60));   /* debounce */
+            xSemaphoreTake(s_touch_sem, 0);  /* coalesce repeats during debounce */
+            handle_touch();
+        }
+    }
+}
+
 void app_on_playback_done(void)
 {
-#if !WIFI_DIAG_MODE
-    face_set(FACE_IDLE); /* in diag mode the status line must stay visible */
-#endif
+    face_set(FACE_IDLE);
 }
 
 void app_main(void)
 {
     ESP_LOGI(TAG, "DeskLock starting");
 
+    s_touch_sem = xSemaphoreCreateBinary();
+
     bsp_display_start();
     bsp_display_backlight_on();
     face_init();
@@ -62,6 +100,8 @@ void app_main(void)
         audio_play_gong(); /* cold boot only — recovery reboots stay silent */
     }
 
+    xTaskCreate(app_task, "app", 4096, NULL, 5, NULL);
+
 #if WIFI_DIAG_MODE
     wifi_diag_start();
 #else