feat(ui): standardize GamepadDebugger empty state with Scan CTA

[ford442]

• Standardized the empty state of GamepadDebugger to match the application's established pattern (dashed borders, circular icon).
• Replaced the text emoji with an SVG icon for better alignment with other panels (e.g., MidiMapPanel, CloudLibrary).
• Added a highly visible, accessible primary call-to-action button ("Scan for Gamepads") that triggers navigator.getGamepads() and provides clear visual feedback (isScanning state) to the user.

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PR created automatically by Jules for task 12707076496177728921 started by @ford442

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