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/ui/ui_manager.h

@@ -1,6 +1,7 @@
 #ifndef UI_MANAGER_H
 #define UI_MANAGER_H
 
+#include <stdbool.h>
 #include <stdint.h>
 #include "lvgl.h"
 
@@ -10,4 +11,11 @@ void ui_init(void);
 void ui_update_weather(int16_t temp_f, const char *condition);
 void ui_set_status(const char *status);
 
+/*
+ * Mark the displayed reading as no longer trustworthy. Dims the temperature
+ * so a value that stopped updating is visibly distinct from a live one,
+ * rather than sitting there at full styling looking current.
+ */
+void ui_set_weather_stale(bool stale);
+
 #endif