Make the game picker's overflow read as scrollable

[austin-smith]

What Changed

• The game picker's scrolling fallback now fades the content at whichever panel edges hide more of it, driven by onScrollGeometryChange, so the fold reads as "scroll for more" instead of an accidental mid-row clip.
• When the picker opens in its scrolling state, it scrolls the current game just into view; an already-visible selection stays put.
• The non-scrolling presentation (windows tall enough to fit every family) is unchanged, as are the family rows themselves.

Why

With ten game families, the gallery overflows a phone screen. The existing ViewThatFits fallback scrolled correctly but gave no cue that it did: the panel clipped Forty Thieves mid-row with no fade or indicator, so the games below the fold — including the newest variants — looked cut off rather than reachable. This also scales to future variants without revisiting the layout.

Validation

• Full test suite on macOS: 541 tests, 0 failures.
• Debug builds succeed for iOS simulator and macOS destinations.
• Verified on the iPhone 17 Pro simulator: bottom fade on open, fade handoff between edges while scrolling, fades releasing at either end, and the picker opening scrolled to Canfield when Canfield is the current game.

UI Changes

• Scrolling picker only: 24pt gradient fades at overflowing panel edges (0.15s ease as they engage/release), and the initial scroll position lands on the current game. No changes to row layout, spacing, or the fits-without-scrolling presentation.

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