Study card: scrollable content + slide-up rating buttons

[bootuz]

Summary

Two study-screen polish improvements on the flip card:

• Scrollable card content (FlipCard.kt → CardFace): long question/answer text now scrolls instead of clipping, while short content stays vertically centered — BoxWithConstraints { Column(verticalScroll().heightIn(min = maxHeight), verticalArrangement = Center) }, the Compose analog of iOS's ScrollView { }.frame(minHeight:). Horizontal padding sits outside the scroll; tap-to-flip and drag-to-scroll coexist; back-face counter-rotation preserved.
• Slide-up rating buttons (StudyScreen.kt → StudyCard): on reveal, the four rating buttons slide up from the bottom + fade in while the "Tap to flip" hint cross-fades. Implemented as a fixed-height Box with two z-stacked Column layers (each hosting an AnimatedVisibility) so the buttons slide over the fading hint rather than being clipped below it.

Also fixes a pre-existing broken instrumented test (StudyContentTest asserted a "Show answer" button that the flip card removed in #8) to use tap-to-flip.

Notes

• Pure presentation — no ViewModel/state/data changes.
• In this Compose version AnimatedVisibility(Boolean) is ColumnScope/RowScope-only (no top-level overload), so the overlap is achieved via two Column(fillMaxSize) layers in a Box.

Test Plan

• :app:testDebugUnitTest passes (no regressions).
• :app:compileDebugKotlin + :app:compileDebugAndroidTestKotlin build clean (instrumented tests compile-verified; emulator unavailable to run them).
• Manual: long card scrolls (drag) / short card centered / tap still flips; on reveal the rating buttons slide up over the fading hint; advancing fades them out and restores the hint.

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