Redesign flag/report icons as vector assets and dock review panels inside their choice card

Flag question now uses a clean vector flag (dark/light SVG variants,
replacing a hand-drawn pixel-art icon that got muddied by the shared
toolbar icon-tint override). Report question is now a megaphone
instead of a second flag glyph, since both toolbar buttons looked
identical before.

The correct-answer explanation and the wrong-answer follow-up quiz
previously rendered as separate bordered boxes sitting below the
entire choice list. Both now dock inside their own choice's card:
the explanation panel reparents onto the correct choice's row, the
follow-up panel was already there but visually detached. A shared
_attached_panel_box style drops the shared border/corner between the
choice button and its panel so they read as one continuous card.

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