feat(sim): board feature parity (reset/GPS/sensors/admin/keyboard) + input fixes

Rounds out the browser sim harness with the rest of the physical board's
interactions: a reset button (JS-driven, since board.reboot() is inert
under -sEXIT_RUNTIME=0), GPS input wired into a real LocationProvider via
new SimSensorManager, JS-settable battery/environment telemetry, an
admin/repeater-login test hook (sendRoomLogin against the default
"password"), and full physical-keyboard text entry (printable ASCII
passthrough into the existing KeyboardWidget, Tab->KEY_KB_ENTER submit).

Also fixes three real bugs found while exercising all of this in a real
browser:
- UITask.cpp's native-only stdin poll branch had no __EMSCRIPTEN__
  exclusion, so it also compiled into the wasm build and called a real,
  blocking window.prompt() on nearly every frame -- the actual cause of
  the reported time/controls jumping. Now gated to native only.
- 'n'/'p' were mapped as Next/Prev keyboard shortcuts, colliding with
  typing those literal letters. Removed the shortcuts; added explicit
  Next/Prev buttons to mesh.html (previously relied solely on them).
- Buttons grabbed native browser keyboard focus on click, so a later
  stray Enter/Space could silently re-trigger a previously-clicked button
  (e.g. Reset). mousedown now calls preventDefault() on all buttons.

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

variants/sim/SimMainBoard.h

@@ -7,10 +7,20 @@
 // mesh::MainBoard implementation for the native sim build -- no real
 // hardware, so battery/manufacturer/reboot are all just plausible fakes.
 class SimMainBoard : public mesh::MainBoard {
+  // Real telemetry code (examples/companion_radio/MyMesh.cpp,
+  // examples/simple_room_server/MyMesh.cpp) reads getBattMilliVolts()
+  // directly rather than going through SensorManager -- so a JS-settable
+  // battery level lives here, not on SimSensorManager. Static (not a plain
+  // member) so the EMSCRIPTEN_KEEPALIVE free function below can reach it
+  // without needing a reference to the one `board` global (target.cpp)
+  // plumbed through, same reasoning as SimLocationProvider.h's singleton.
+  static uint16_t& battMilliVoltsRef() { static uint16_t mv = 4000; return mv; }
+
 public:
   void begin() { }
 
-  uint16_t getBattMilliVolts() override { return 4000; }  // pretend full battery
+  uint16_t getBattMilliVolts() override { return battMilliVoltsRef(); }
+  static void setBattMilliVolts(uint16_t mv) { battMilliVoltsRef() = mv; }
   const char* getManufacturerName() const override { return "MeshCore Sim (native)"; }
 
   void reboot() override {
@@ -24,3 +34,13 @@ public:
   void onBootComplete() override { }
   void sleep(uint32_t secs) override { }   // no-op: native process never actually sleeps the CPU
 };
+
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+// C linkage so JS's ccall('sim_battery_set_mv', ...) can find it by its
+// exact, unmangled name -- same pattern as SimLocationProvider.h's
+// sim_location_set().
+extern "C" inline EMSCRIPTEN_KEEPALIVE void sim_battery_set_mv(int mv) {
+  SimMainBoard::setBattMilliVolts((uint16_t)mv);
+}
+#endif