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.

assets/icons/medmind/flag-dark.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none">
+  <rect x="5" y="3.4" width="1.6" height="17.2" rx="0.8" fill="#F6F8FF"/>
+  <path d="M6.6 4.8H17.3L14.6 8.4L17.3 12H6.6V4.8Z" fill="#FFC857"/>
+</svg>