fix(study): the explanation has an open book, the hints the light bulb

Both pills showed a light bulb. The explanation's now has the open book
that Add explanation has in the composer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/src/feature/card/widget/card_extras.dart

@@ -12,6 +12,7 @@ class const HintsButton({required final List<RichContent> hints, super.key})
   Widget build(BuildContext context) => hints.isEmpty
       ? const SizedBox.shrink()
       : _Pill(
+          icon: LucideIcons.lightbulb,
           label: AppLocalizations.of(context).cardHints,
           onPressed: () => showAdaptiveSheet<void>(
             context,
@@ -20,7 +21,9 @@ class const HintsButton({required final List<RichContent> hints, super.key})
         );
 }
 
-/// A card's [explanation] as a pill to open it; nothing without.
+/// A card's [explanation] as a pill to open it; nothing without. An open
+/// book, as Add explanation has in the composer, tells it from the hints'
+/// light bulb.
 class const ExplanationButton({
   required final RichContent? explanation,
   super.key,
@@ -30,6 +33,7 @@ class const ExplanationButton({
     final l10n = AppLocalizations.of(context);
     return switch (explanation) {
       final explanation? => _Pill(
+        icon: LucideIcons.bookOpen,
         label: l10n.cardExplanation,
         onPressed: () => showAdaptiveSheet<void>(
           context,
@@ -44,8 +48,9 @@ class const ExplanationButton({
   }
 }
 
-/// A light bulb and [label] on an outlined pill, in muted colours.
+/// [icon] and [label] on an outlined pill, in muted colours.
 class const _Pill({
+  required final IconData icon,
   required final String label,
   required final VoidCallback onPressed,
 }) extends StatelessWidget {
@@ -54,7 +59,7 @@ class const _Pill({
     final colors = Theme.of(context).colorScheme;
     return OutlinedButton.icon(
       onPressed: onPressed,
-      icon: const Icon(LucideIcons.lightbulb, size: 16),
+      icon: Icon(icon, size: 16),
       label: Text(label),
       style: OutlinedButton.styleFrom(
         foregroundColor: colors.onSurfaceVariant,

halkyon_learn_flutter/test/feature/study/widget/study_screen_test.dart

@@ -304,7 +304,8 @@ void main() {
   });
 
   testWidgets('hints open from under the question, one at a time; the '
-      'explanation comes with the answer', (tester) async {
+      "explanation comes with the answer, an open book to the hints' light "
+      'bulb', (tester) async {
     await cards.create(
       'japanese',
       CardContent(
@@ -330,6 +331,14 @@ void main() {
 
     await tester.tap(find.text('Show answer'));
     await tester.pumpAndSettle();
+    expect(
+      find.widgetWithIcon(OutlinedButton, LucideIcons.lightbulb),
+      findsOneWidget,
+    );
+    expect(
+      find.widgetWithIcon(OutlinedButton, LucideIcons.bookOpen),
+      findsOneWidget,
+    );
     await tester.tap(find.text('Explanation'));
     await tester.pumpAndSettle();
     expect(