Fix TWDT starvation, vsync race, and UI responsiveness from review

Three bugs and two UX defects found on review, plus hardening:

- The control loop fed the task watchdog then slept exactly the watchdog
  period (both 10s), tripping the TWDT fractionally late on every single
  iteration. Sleep is now sliced 1s with a reset per slice. Also set
  CONFIG_ESP_TASK_WDT_PANIC=y - the sdkconfig comment claimed a hung task
  panics the chip, but nothing actually made that true.

- The LVGL flush called draw_bitmap (which only queues the framebuffer
  switch) and released LVGL immediately, letting it render the next frame
  into the buffer still being scanned out - the exact PRD 8.2 race the
  double-buffer design exists to prevent. display_present_frame() now
  blocks on a vsync handshake until the swap has latched.

- WiFi reconnect slept up to 30s inside the system event handler, stalling
  all queued WiFi/IP events. Moved to an esp_timer.

- The setpoint label, arc, and mode/room lines rendered the control loop's
  copy of state, which refreshes every 10s - every knob detent looked dead
  for up to 10 seconds. The UI now reads settings live; when the fallback
  ladder clamps the setpoint it shows both values. Room-select focus now
  shows which room is being selected.

- z2m ISO_8601_local timestamps carry a UTC offset the parser ignored,
  which would shift apparent reading age by the local offset (5-6h in the
  US) and permanently trip the fallback ladder. Offset is now honoured.

- Suppressed the cosmetic "control loop stopped" fault in the window
  between relay_init and the control task's first call-in at boot.

- Explicit pullups on the encoder A/B lines.

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

sdkconfig.defaults

@@ -71,8 +71,13 @@ CONFIG_GDMA_CTRL_FUNC_IN_IRAM=y
 # --- Watchdogs ---------------------------------------------------------------
 # The task watchdog is part of the safety story: a hung control task panics the
 # chip, and the reboot path de-asserts the relay within about a second.
+# ESP_TASK_WDT_PANIC is what makes that true — without it the TWDT only logs a
+# warning and a hung task keeps hanging. (The relay layer's own 30s watchdog
+# has already dropped the heat call by then; this reboots the chip so the
+# thermostat comes back instead of staying wedged.)
 CONFIG_ESP_TASK_WDT_INIT=y
 CONFIG_ESP_TASK_WDT_TIMEOUT_S=10
+CONFIG_ESP_TASK_WDT_PANIC=y
 CONFIG_ESP_TASK_WDT_CHECK_IDLE_TASK_CPU0=n
 CONFIG_ESP_TASK_WDT_CHECK_IDLE_TASK_CPU1=n
 CONFIG_ESP_SYSTEM_PANIC_PRINT_REBOOT=y