feat(display): add R8 observer TFT dashboard, touch toggle and display.timeout

Replace the sparse Heltec V4 R8 observer home screen with a padded dark
analytics dashboard, add manual display control, and make blanking a runtime
setting.

Dashboard (DISPLAY_ACTIVITY_DASHBOARD, the four R8 TFT observer envs):

- RadioActivityWindow: 20 one-minute buckets of valid RX packets, no heap.
  The caller's 32-bit millis() is extended to a monotonic 64-bit clock, so
  nothing downstream has a rollover case; an always-on node passes 2^32 ms
  after ~49.7 days, which would otherwise re-enter warm-up and divide 20
  minutes of traffic by seconds. Rates use 19 whole minutes plus the elapsed
  part of the current one rather than a fixed 1200 s.
- ObserverDashboard: header, radio strip, headline totals, a 20-bar
  packets-per-minute graph and RF/status footers, with separate portrait and
  landscape layouts. A text row is a fixed 16 px, which is 3.2 logical units
  in portrait but 4.27 in landscape, so one shared grid would overlap.
  Text is trimmed by character budget, not measured width: getTextWidth()
  reports an over-long string at the portrait driver's fallback scale, so
  DisplayDriver::drawTextEllipsized() under-trims and the row renders at half
  height.
- Six per-row signatures computed from what is actually drawn, so only the
  rows whose pixels changed repaint. No startFrame(), no whole-screen clear.
  Link state moved out of the full-frame signature, so a DHCP renewal or WiFi
  flap repaints one footer row instead of the panel.
- Dark theme by retuning the UIColor statics at runtime, which needs no
  display-driver edit and carries boot, setup, reboot and power-off with it.

Touch and button (DISPLAY_TOUCH_TOGGLE):

- CHSC6X at I2C 0x2E, polled; TP_INT is unusable (optional R13, and GPIO 43
  is U0TXD). The point-count byte is tested against a valid count, never
  against non-zero: an idle read returns 0xFF, which reads as a finger held
  down forever and latches the tap detector after one event.
- turnOff() no longer parks PIN_TFT_RST low on this board. GPIO 21 is a
  shared LCD_RST/TP_RST net, so doing that held the touch controller in
  reset for as long as the display was off. Verified against Heltec's
  expansion-board and mainboard schematics and the V4-R8 datasheet pinout,
  which also correct the pin comment in HeltecV4R8Board.cpp.
- The USER button click now toggles the display too; it previously did
  nothing whenever the display was already on.

display.timeout:

- `set display.timeout <secs>` / `get display.timeout`, 0 = stay on, 60 s
  default, 3600 max. Read live, so a change applies without a reboot and
  restarts the countdown rather than firing on the old deadline.
- Stored in MQTTPrefs (/mqtt.json), keeping NodePrefs aligned with upstream.
  Runtime-only: LegacyV1MQTTPrefs and the four frozen binary payload sizes
  are unchanged. No JSON format-version bump - the loader skips keys no
  def() claims, so older firmware reads newer files and this firmware reads
  older ones with the default applied. Both directions are covered by tests.
- Joins the observer atomic-setter contract, so a failed save rolls the live
  value back instead of only claiming to.

New periodic work uses a wrap-safe deadline check; `millis() >= deadline`
fires every loop for a whole interval before each rollover.

Adds test_radio_activity_window, test_observer_dashboard (driving the real
renderer against a recording DisplayDriver in both orientation profiles) and
test_touch_tap_detector. 440 native cases pass.

examples/simple_room_server/main.cpp

@@ -91,7 +91,13 @@ void setup() {
 
 #ifdef DISPLAY_CLASS
   if (display_ready) {
+#ifdef WITH_MQTT_BRIDGE
+    ui_task.setObserverPrefs(the_mesh.getObserverPrefs());
+#endif
     ui_task.begin(the_mesh.getNodePrefs(), FIRMWARE_BUILD_DATE, FIRMWARE_VERSION);
+#ifdef DISPLAY_ACTIVITY_DASHBOARD
+    ui_task.setActivityWindow(the_mesh.getActivityWindow());
+#endif
   }
 #endif