Work through the remaining review items in TODO.md

Keep the HTTP session alive between weather polls (TODO #1). Re-creating the
client each time forced a fresh TLS handshake inside the window where the UI
is frozen. Measured on hardware: the fetch went from ~1.3s to ~80ms, so the
pause is no longer long enough to matter once thermostat controls exist. The
timeout drops from 30s to 10s once a session is warm, since only a cold
handshake needs the longer budget. Any transport error or non-200 tears the
session down so a half-open connection cannot poison every later poll.

The response buffer is now reset per request rather than on
HTTP_EVENT_ON_CONNECTED -- a kept-alive connection does not re-fire that
event, so the buffer would have accumulated across polls.

Indicate stale readings (TODO #2). After two consecutive failures the
temperature dims and the status line shows the age. Age comes from tick
counts, not weather_data_t.last_update: there is no SNTP client here, so
time(NULL) starts at the epoch and any age derived from it is meaningless.

Gate the touch readout behind UI_DEBUG_TOUCH, default off (TODO #3). Kept the
note about LVGL not bubbling events to parents -- it documents a real trap.

Delete main/utils (TODO #4); it had no callers outside itself. Also refreshed
project_structure.txt, which was additionally missing i2c_bus and backlight.

Lay out two 4MB OTA slots (TODO #5) while no devices are fielded, and document
in the README that an OTA download cannot run while the panel is scanning --
the flash write faults the bounce-buffer ISR.

Log the truncation warning once per response instead of once per chunk, and
record the weather task's stack high-water mark (TODO #6): ~2400 bytes spare
of 6144.

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

main/main.c

@@ -18,15 +18,36 @@
 
 static const char *TAG = "main";
 
+/*
+ * 6144 bytes. Measured headroom after a fetch including the TLS handshake --
+ * the deepest this task gets -- is ~2400 bytes, so roughly 3.7KB is in use.
+ * mbedTLS keeps its heap in PSRAM (CONFIG_MBEDTLS_EXTERNAL_MEM_ALLOC), which
+ * is why the handshake fits at all. Re-check the high-water-mark log if more
+ * work lands in this task.
+ */
 #define WEATHER_TASK_STACK 6144
 #define WEATHER_TASK_PRIO  3
 #define WEATHER_FIRST_DELAY_MS 3000
 
+/* Consecutive failed polls before the displayed reading is treated as stale. */
+#define WEATHER_STALE_AFTER_POLLS 2
+
 static void weather_update_task(void *arg)
 {
     weather_data_t weather;
     char line[96];
 
+    /*
+     * Age is tracked in ticks, not from weather_data_t.last_update. That field
+     * comes from time(NULL), and with no SNTP client the system clock starts
+     * at the epoch -- an age derived from it would be nonsense. Tick
+     * subtraction is also wrap-safe.
+     */
+    TickType_t last_success = 0;
+    int consecutive_failures = 0;
+    bool have_reading = false;
+    bool stack_logged = false;
+
     vTaskDelay(pdMS_TO_TICKS(WEATHER_FIRST_DELAY_MS));
 
     while (1) {
@@ -48,6 +69,10 @@ static void weather_update_task(void *arg)
         lvgl_port_resume();
 
         if (err == ESP_OK) {
+            last_success = xTaskGetTickCount();
+            consecutive_failures = 0;
+            have_reading = true;
+
             weather_get_string(&weather, line, sizeof(line));
             ESP_LOGI(TAG, "Weather: %s", line);
 
@@ -59,12 +84,31 @@ static void weather_update_task(void *arg)
              * below 0°F, turning -10.6 into -10. */
             ui_update_weather((int16_t)lroundf(weather.temp_f),
                               weather.has_humidity ? detail : "Outdoor");
+            ui_set_weather_stale(false);
 
             wifi_get_status(line, sizeof(line));
             ui_set_status(line);
+
+            /* One-shot, after a fetch that included a TLS handshake -- that is
+             * the deepest this task's stack ever gets. */
+            if (!stack_logged) {
+                stack_logged = true;
+                ESP_LOGI(TAG, "weather task stack headroom: %u bytes",
+                         (unsigned)(uxTaskGetStackHighWaterMark(NULL) * sizeof(StackType_t)));
+            }
         } else {
-            ESP_LOGW(TAG, "Weather update failed");
-            ui_set_status("Weather update failed");
+            consecutive_failures++;
+            ESP_LOGW(TAG, "Weather update failed (%d consecutive)", consecutive_failures);
+
+            if (have_reading && consecutive_failures >= WEATHER_STALE_AFTER_POLLS) {
+                const uint32_t age_min =
+                    ((xTaskGetTickCount() - last_success) * portTICK_PERIOD_MS) / 60000U;
+                ui_set_weather_stale(true);
+                snprintf(line, sizeof(line), "No update for %lum", (unsigned long)age_min);
+                ui_set_status(line);
+            } else {
+                ui_set_status("Weather update failed");
+            }
         }
 
         vTaskDelay(pdMS_TO_TICKS(WEATHER_UPDATE_INTERVAL_MS));