refactor: mehrstufiger Quality-Filter statt Toggle (#24)

Statt einfachem On/Off Toggle jetzt 4 Qualitätsstufen:
- 📊 Alle (kein Filter)
- ✓ Hochwertig (excellent + good + acceptable)
- ✓✓ Sehr gut (excellent + good)
- ⭐ Exzellent (nur excellent)

UI:
- Button-Group (Segmented Control) mit 4 Stufen
- Beschreibung welche Labels inkludiert werden
- Anzeige: X von Y Aktivitäten (wenn gefiltert)

User-Feedback: Stufenweiser Filter ist flexibler als binärer Toggle

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/History.jsx

@@ -588,16 +588,22 @@ function NutritionSection({ nutrition, weights, profile, insights, onRequest, lo
 // ── Activity Section ──────────────────────────────────────────────────────────
 function ActivitySection({ activities, insights, onRequest, loadingSlug, filterActiveSlugs }) {
   const [period, setPeriod] = useState(30)
-  const [qualityFilter, setQualityFilter] = useState(false) // Issue #24: Quality-Filter Toggle
+  const [qualityLevel, setQualityLevel] = useState('all') // Issue #24: Quality-Filter (all, quality, very_good, excellent)
   if (!activities?.length) return (
     <EmptySection text="Noch keine Aktivitätsdaten." to="/activity" toLabel="Aktivität erfassen"/>
   )
   const cutoff = dayjs().subtract(period,'day').format('YYYY-MM-DD')
-  // Issue #24: Filter nach Datum UND Quality-Label
-  const filtA  = activities.filter(d =>
-    (period===9999 || d.date>=cutoff) &&
-    (!qualityFilter || ['excellent', 'good', 'acceptable'].includes(d.quality_label))
-  )
+
+  // Issue #24: Mehrstufiger Quality-Filter
+  const filtA = activities.filter(d => {
+    if (period !== 9999 && d.date < cutoff) return false
+
+    if (qualityLevel === 'all') return true
+    if (qualityLevel === 'quality') return ['excellent', 'good', 'acceptable'].includes(d.quality_label)
+    if (qualityLevel === 'very_good') return ['excellent', 'good'].includes(d.quality_label)
+    if (qualityLevel === 'excellent') return d.quality_label === 'excellent'
+    return true
+  })
 
   const byDate={}
   filtA.forEach(a=>{ byDate[a.date]=(byDate[a.date]||0)+(a.kcal_active||0) })
@@ -626,20 +632,34 @@ function ActivitySection({ activities, insights, onRequest, loadingSlug, filterA
       <SectionHeader title="🏋️ Aktivität" to="/activity" toLabel="Alle Einträge" lastUpdated={activities[0]?.date}/>
       <PeriodSelector value={period} onChange={setPeriod}/>
 
-      {/* Issue #24: Quality-Filter Toggle */}
-      <div style={{marginBottom:12,display:'flex',alignItems:'center',gap:8}}>
-        <label style={{display:'flex',alignItems:'center',gap:6,fontSize:12,cursor:'pointer',
-          padding:'6px 10px',borderRadius:8,background:qualityFilter?'var(--accent-light)':'var(--surface2)',
-          border:`1px solid ${qualityFilter?'var(--accent)':'var(--border)'}`,
-          color:qualityFilter?'var(--accent)':'var(--text2)',transition:'all 0.2s'}}>
-          <input type="checkbox" checked={qualityFilter} onChange={e=>setQualityFilter(e.target.checked)}
-            style={{width:14,height:14,cursor:'pointer',accentColor:'var(--accent)'}}/>
-          <span style={{fontWeight:qualityFilter?600:400}}>Nur qualitativ hochwertige Aktivitäten</span>
-        </label>
-        {qualityFilter && (
-          <span style={{fontSize:11,color:'var(--text3)'}}>
-            ({filtA.length} von {activities.filter(d=>period===9999||d.date>=cutoff).length})
-          </span>
+      {/* Issue #24: Mehrstufiger Quality-Filter */}
+      <div style={{marginBottom:12}}>
+        <div style={{fontSize:11,fontWeight:600,color:'var(--text3)',marginBottom:6}}>QUALITÄTSFILTER</div>
+        <div style={{display:'flex',gap:4}}>
+          {[
+            {v:'all', l:'Alle', icon:'📊'},
+            {v:'quality', l:'Hochwertig', icon:'✓'},
+            {v:'very_good', l:'Sehr gut', icon:'✓✓'},
+            {v:'excellent', l:'Exzellent', icon:'⭐'}
+          ].map(o => (
+            <button key={o.v} onClick={() => setQualityLevel(o.v)}
+              style={{flex:1,padding:'6px 8px',borderRadius:8,fontSize:11,fontWeight:500,
+                border:'1.5px solid',cursor:'pointer',fontFamily:'var(--font)',transition:'all 0.2s',
+                background:qualityLevel===o.v?'var(--accent)':'transparent',
+                borderColor:qualityLevel===o.v?'var(--accent)':'var(--border2)',
+                color:qualityLevel===o.v?'white':'var(--text2)'}}>
+              <span style={{marginRight:4}}>{o.icon}</span>
+              {o.l}
+            </button>
+          ))}
+        </div>
+        {qualityLevel !== 'all' && (
+          <div style={{fontSize:10,color:'var(--text3)',marginTop:6}}>
+            {filtA.length} von {activities.filter(d=>period===9999||d.date>=cutoff).length} Aktivitäten
+            {qualityLevel === 'quality' && ' (excellent, good, acceptable)'}
+            {qualityLevel === 'very_good' && ' (excellent, good)'}
+            {qualityLevel === 'excellent' && ' (nur excellent)'}
+          </div>
         )}
       </div>