fix(controller): guard btnHome against gamepads without a buttons array

[Endymi0n74]

Fixes the crash class reported in #991: controller input silently stops working — pollGamepad throws Cannot read properties of undefined (reading 'pressed').

Root cause

poll-gamepad reads the Home button state on every poll:

const btnHome = currentGamepad.buttons[16];

Gamepad objects are not guaranteed to expose a buttons array at every poll (observed on a GameSir G7 SE). When the array is missing, buttons[16] throws, the exception kills the whole polling loop and controller input silently stops working until a reload — the exact symptom in #991.

Fix

const btnHome = currentGamepad.buttons?.[16];

• When buttons is missing, btnHome is undefined → the controller-shortcuts block becomes a no-op.
• The rest of the poll (gamepad input delivery, button mapping) keeps running.
• Zero behavioral change for gamepads that report buttons normally.

Verification

• Build: exit 0 (eslint + TS gate).
• Bundle: guarded access present, unguarded buttons[16] gone.
• Minimal diff: 1 file, +3/−1.

@redphx — happy to adjust if you'd rather guard currentGamepad itself too.