Add WiFi reset button via GPIO 0 (BOOT button)

[matt1394]

Summary

• Adds hold-to-reset WiFi functionality using the built-in BOOT button (GPIO 0)
• Holding the button for 3 seconds wipes saved credentials and reboots into the captive portal setup AP
• Releasing before 3 seconds cancels the reset
• Documents the button in README under Hardware GPIO mapping and a new "Resetting WiFi Credentials" section

Test plan

• Flash firmware and confirm normal boot is unaffected (brief BOOT button press does nothing)
• Hold BOOT button for <3 seconds and confirm reset is cancelled (check Serial output)
• Hold BOOT button for 3+ seconds and confirm device reboots into Rotor-Controller-Setup AP
• Reconnect to new WiFi via captive portal and confirm normal operation resumes

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