feat(exercise): add support for exercise variants in ExerciseFullContent and related components

- Updated ExerciseFullContent to accept a new `variantId` prop and display variant-specific information.
- Enhanced TrainingCoachPage to pass `variantId` when rendering ExerciseFullContent.
- Refactored TrainingUnitRunPage to manage exercise context with variant support, including updates to modal handling.
- Improved UI to show variant details, including name, description, and execution changes where applicable.

frontend/src/components/ExerciseFullContent.jsx

@@ -1,5 +1,6 @@
 /**
  * Voller Katalog-Inhalt einer Übung (Lesemodus für Coach/Mobile).
+ * Optional: geplante Variante (`variantId`) — Beschreibung und Durchführungsänderungen oben.
  */
 import React from 'react'
 import { Link } from 'react-router-dom'
@@ -51,9 +52,9 @@ function metaParts(exercise) {
 }
 
 /**
- * @param {{ exercise?: object|null, loading?: boolean, error?: string|null, exerciseId?: number }} props
+ * @param {{ exercise?: object|null, loading?: boolean, error?: string|null, exerciseId?: number, variantId?: number|string|null }} props
  */
-export default function ExerciseFullContent({ exercise, loading, error, exerciseId }) {
+export default function ExerciseFullContent({ exercise, loading, error, exerciseId, variantId }) {
   if (loading) {
     return (
       <div style={{ textAlign: 'center', padding: '1rem' }}>
@@ -70,8 +71,41 @@ export default function ExerciseFullContent({ exercise, loading, error, exercise
   const resolvedId = exercise.id ?? exerciseId
   const meta = metaParts(exercise)
 
+  const variant =
+    variantId != null && variantId !== '' && Array.isArray(exercise.variants) && exercise.variants.length
+      ? exercise.variants.find((v) => String(v.id) === String(variantId)) || null
+      : null
+
   return (
     <div className="exercise-coach-catalog" style={{ fontSize: '0.93rem', lineHeight: 1.5 }}>
+      {variant ? (
+        <section
+          className="card"
+          style={{
+            marginBottom: '14px',
+            padding: '12px 14px',
+            borderLeft: '3px solid var(--accent)',
+            background: 'var(--surface2)',
+          }}
+        >
+          <h3 style={{ fontSize: '0.72rem', textTransform: 'uppercase', color: 'var(--text3)', margin: '0 0 6px', letterSpacing: '0.04em' }}>
+            Geplante Variante
+          </h3>
+          <p style={{ margin: '0 0 10px', fontSize: '1.05rem', fontWeight: 700 }}>{variant.variant_name || `Variante #${variant.id}`}</p>
+          {variant.description ? (
+            <div style={{ marginBottom: variant.execution_changes ? '12px' : 0 }}>
+              <h4 style={{ fontSize: '0.78rem', textTransform: 'uppercase', color: 'var(--text3)', marginBottom: '8px' }}>Zur Variante</h4>
+              <ExerciseRichTextBlock html={variant.description} exerciseId={resolvedId} media={exercise.media} />
+            </div>
+          ) : null}
+          {variant.execution_changes ? (
+            <div>
+              <h4 style={{ fontSize: '0.78rem', textTransform: 'uppercase', color: 'var(--text3)', marginBottom: '8px' }}>Durchführung (Variante)</h4>
+              <ExerciseRichTextBlock html={variant.execution_changes} exerciseId={resolvedId} media={exercise.media} />
+            </div>
+          ) : null}
+        </section>
+      ) : null}
       <h2 style={{ margin: '0 0 8px', fontSize: '1.2rem', lineHeight: 1.35 }}>{exercise.title}</h2>
       {meta.length > 0 && (
         <p className="exercise-meta-line" style={{ marginBottom: '10px', color: 'var(--text3)', fontSize: '0.86rem' }}>

frontend/src/pages/TrainingCoachPage.jsx

@@ -400,7 +400,7 @@ export default function TrainingCoachPage() {
     return () => {
       cancelled = true
     }
-  }, [step, currentEntry?.item?.exercise_id, currentEntry?.item?.item_type])
+  }, [step, currentEntry?.item?.exercise_id, currentEntry?.item?.exercise_variant_id, currentEntry?.item?.item_type])
 
   const handleSaveDebrief = async () => {
     setSaveOk(null)
@@ -739,6 +739,7 @@ export default function TrainingCoachPage() {
                     error={catalogError}
                     exercise={catalogExercise}
                     exerciseId={currentEntry?.item?.exercise_id ?? null}
+                    variantId={currentEntry?.item?.exercise_variant_id ?? null}
                   />
                 </div>
               </>

frontend/src/pages/TrainingUnitRunPage.jsx

@@ -33,7 +33,7 @@ export default function TrainingUnitRunPage() {
   const [loadError, setLoadError] = useState(null)
   const [loading, setLoading] = useState(true)
   const [checked, setChecked] = useState(() => new Set())
-  const [peekExerciseId, setPeekExerciseId] = useState(null)
+  const [peekCtx, setPeekCtx] = useState(null)
 
   const loadChecked = useCallback((uid) => {
     try {
@@ -143,9 +143,10 @@ export default function TrainingUnitRunPage() {
   return (
     <div className="training-run-page app-page" style={{ paddingBottom: '2rem' }}>
       <ExercisePeekModal
-        open={peekExerciseId != null}
-        exerciseId={peekExerciseId}
-        onClose={() => setPeekExerciseId(null)}
+        open={peekCtx != null}
+        exerciseId={peekCtx?.exerciseId}
+        variantId={peekCtx?.variantId ?? undefined}
+        onClose={() => setPeekCtx(null)}
       />
 
       <nav className="training-run-toolbar no-print" style={{ marginBottom: '1rem', display: 'flex', gap: '0.5rem', flexWrap: 'wrap', alignItems: 'center' }}>
@@ -314,7 +315,12 @@ export default function TrainingUnitRunPage() {
                                   type="button"
                                   className="btn btn-secondary"
                                   style={{ fontSize: '0.82rem', margin: 0 }}
-                                  onClick={() => setPeekExerciseId(it.exercise_id)}
+                                  onClick={() =>
+                                    setPeekCtx({
+                                      exerciseId: it.exercise_id,
+                                      variantId: it.exercise_variant_id != null ? Number(it.exercise_variant_id) : null,
+                                    })
+                                  }
                                 >
                                   Katalog (Popup)
                                 </button>