fix: dynamically assign button pin to prevent false interrupts

[LN4CY]

This PR resolves the false interrupt and random wake issues caused by floating button pins on variants where the button is not physically wired (such as rak3401_1watt), while fully restoring button functionality when a GPIO is explicitly assigned via the Meshtastic configuration.

Changes Made:

• Removed hardcoded #ifdef BUTTON_PIN guards in InputBroker.cpp, PowerFSM.cpp, sleep.cpp, and main-esp32.cpp in favor of #if HAS_BUTTON.
• Refactored logic to dynamically resolve _btnPin using the following priority:
  1. config.device.button_gpio (if explicitly assigned at runtime)
  2. USERPREFS_BUTTON_PIN (if defined in userprefs)
  3. BUTTON_PIN (if defined in the variant header)
• If no valid pin is resolved (evaluates to 0xFF), the firmware skips configuring the button pull-ups, skips initializing ButtonThread, and ignores digital reads during sleep, avoiding any false positives on floating pins.

?? Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
  • Seeed Studio T-1000E tracker card
  • Other: RAK3401 1-Watt
  • Other: RAK WisMesh Tap

Fixes #10574

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