feat: add trial system UI with countdown banner

Component:
- TrialBanner.jsx: Displays remaining trial days with urgency levels

Features:
- Calculates days left from profile.trial_ends_at
- Three urgency levels:
  * Normal (>7 days): Accent blue, "Abo wählen"
  * Warning (≤7 days): Orange, "Abo wählen"
  * Urgent (≤3 days): Red + ⚠️, "Jetzt upgraden"
- Auto-hides when no trial or trial ended
- Responsive flex layout
- Call-to-action button links to /settings?tab=subscription

Integration:
- Added to Dashboard after header greeting
- Uses activeProfile from ProfileContext
- Clean, non-intrusive design

UX:
- Clear messaging: "Trial endet in X Tagen"
- Special case: "morgen" for 1 day left
- Color-coded severity (blue → orange → red)
- Prominent CTA button

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

frontend/src/components/TrialBanner.jsx

@@ -0,0 +1,86 @@
+import { useEffect, useState } from 'react'
+import { Link } from 'react-router-dom'
+
+export default function TrialBanner({ profile }) {
+  const [daysLeft, setDaysLeft] = useState(null)
+
+  useEffect(() => {
+    if (!profile?.trial_ends_at) {
+      setDaysLeft(null)
+      return
+    }
+
+    const trialEnd = new Date(profile.trial_ends_at)
+    const now = new Date()
+    const diff = trialEnd - now
+    const days = Math.ceil(diff / (1000 * 60 * 60 * 24))
+
+    setDaysLeft(days)
+  }, [profile])
+
+  // No trial or trial ended
+  if (daysLeft === null || daysLeft <= 0) return null
+
+  // Determine severity
+  const isUrgent = daysLeft <= 3
+  const isWarning = daysLeft <= 7
+
+  const bgColor = isUrgent ? '#FCEBEB' : isWarning ? '#FFF4E6' : 'var(--accent-light)'
+  const borderColor = isUrgent ? '#D85A30' : isWarning ? '#F59E0B' : 'var(--accent)'
+  const textColor = isUrgent ? '#D85A30' : isWarning ? '#D97706' : 'var(--accent-dark)'
+
+  return (
+    <div style={{
+      background: bgColor,
+      border: `2px solid ${borderColor}`,
+      borderRadius: 12,
+      padding: '16px 20px',
+      marginBottom: 20,
+      display: 'flex',
+      alignItems: 'center',
+      justifyContent: 'space-between',
+      flexWrap: 'wrap',
+      gap: 12
+    }}>
+      <div style={{flex: 1, minWidth: 200}}>
+        <div style={{
+          fontWeight: 700,
+          fontSize: 15,
+          color: textColor,
+          marginBottom: 4
+        }}>
+          {isUrgent && '⚠️ '}
+          Deine Trial endet {daysLeft === 1 ? 'morgen' : `in ${daysLeft} Tagen`}
+        </div>
+        <div style={{
+          fontSize: 13,
+          color: 'var(--text2)',
+          lineHeight: 1.4
+        }}>
+          {isUrgent
+            ? 'Upgrade jetzt um weiterhin alle Features nutzen zu können'
+            : 'Wähle ein Abo um unbegrenzt Zugriff zu erhalten'
+          }
+        </div>
+      </div>
+
+      <Link
+        to="/settings?tab=subscription"
+        style={{
+          padding: '10px 20px',
+          borderRadius: 8,
+          background: isUrgent ? '#D85A30' : 'var(--accent)',
+          color: 'white',
+          fontWeight: 600,
+          fontSize: 14,
+          textDecoration: 'none',
+          whiteSpace: 'nowrap',
+          border: 'none',
+          cursor: 'pointer'
+        }}
+      >
+        {isUrgent ? 'Jetzt upgraden' : 'Abo wählen'}
+      </Link>
+    </div>
+  )
+}

frontend/src/pages/Dashboard.jsx

@@ -8,6 +8,7 @@ import {
 import { api } from '../utils/api'
 import { useProfile } from '../context/ProfileContext'
 import { getBfCategory } from '../utils/calc'
+import TrialBanner from '../components/TrialBanner'
 import { getInterpretation, getStatusColor, getStatusBg } from '../utils/interpret'
 import Markdown from '../utils/Markdown'
 import dayjs from 'dayjs'
@@ -315,6 +316,9 @@ export default function Dashboard() {
         </div>
       </div>
 
+      {/* Trial Banner */}
+      <TrialBanner profile={activeProfile}/>
+
       {!hasAnyData && (
         <div className="empty-state">
           <h3>Willkommen bei Mitai Jinkendo!</h3>