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/SimLocationProvider.h

@@ -49,12 +49,15 @@ public:
 };
 
 // Single instance, mirroring the pattern of every other Sim* global
-// (board/radio_driver/rtc_clock/sensors) declared in target.h/target.cpp --
-// this one isn't wired into target.h/.cpp itself since nothing on the
-// SIM_PLATFORM boot path currently constructs a LocationProvider at all
-// (see the class comment above), so it lives here instead as a
-// self-contained, opt-in addition.
-extern SensorManager sensors;   // defined in variants/sim/target.cpp
+// (board/radio_driver/rtc_clock/sensors) declared in target.h/target.cpp.
+// Typed as SimSensorManager, not the SensorManager base, because C++
+// requires every declaration of the same global variable to agree on its
+// exact type -- target.cpp's actual `SimSensorManager sensors;` definition
+// would otherwise conflict with a base-typed extern here. This only
+// compiles because target.h includes SimSensorManager.h (which declares
+// the class) before this file -- see target.h's own comment on that
+// ordering.
+extern SimSensorManager sensors;   // defined in variants/sim/target.cpp
 inline SimLocationProvider& sim_location_provider() {
   static SimLocationProvider instance;
   return instance;