fix(v2): stop gamepad B/cancel from backing out of controller test and games

[gantoine]

The global useGamepad loop translated every button into a built-in action regardless of context. On the controller-test screen this meant pressing B (or Circle / Nintendo-A, which share the standard B index) fired "back" and left the page before the press could be inspected. The same happened while a game was running: the emulator owns the pad, but useGamepad still ran "back" on B and quit the game.

Suppress the built-in translation in two contexts:

• While a game is running (storePlaying.playing), so the emulator owns the pad entirely.
• On the controller-test route, so every button can be pressed and inspected in place.

Because the test screen no longer exits on B, ControllerDebug owns its own escape: hold a back button (B / Circle or Back / Share) for 700ms to navigate away, with an on-screen hint and a fill indicator. A short tap just registers in the inspector.

Fixes #3591

Claude-Session: https://claude.ai/code/session_01EdiAR2HMM5pCMVfn2Zhbrx

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[gantoine]

@zurdi15 claude took a crack at #3591, i no longer have a usb gamepad, can you test this one and push changes if needed?

[zurdi15]

@zurdi15 claude took a crack at #3591, i no longer have a usb gamepad, can you test this one and push changes if needed?

Sure I'll approve once I can review it