feat: placeholder chips + convert to base prompt (Issue #28)

New features:
1. Placeholder chips now visible in pipeline inline templates
   - Click to insert: weight_data, nutrition_data, activity_data, etc.
   - Same UX as base prompts

2. Convert to Base Prompt button
   - New icon (ArrowDownToLine) in actions column
   - Only visible for 1-stage pipeline prompts
   - Converts pipeline → base by extracting inline template
   - Validates: must be 1-stage, 1-prompt, inline source

This allows migrated prompts to be properly categorized as base prompts
for reuse in other pipelines.

frontend/src/components/UnifiedPromptModal.jsx

@@ -526,14 +526,41 @@ export default function UnifiedPromptModal({ prompt, onSave, onClose }) {
                             ))}
                         </select>
                       ) : (
-                        <textarea
-                          className="form-input"
-                          value={p.template || ''}
-                          onChange={e => updateStagePrompt(stage.stage, pIdx, 'template', e.target.value)}
-                          rows={3}
-                          placeholder="Inline-Template mit {{placeholders}}..."
-                          style={{ width: '100%', fontSize: 12, textAlign: 'left', resize: 'vertical', fontFamily: 'monospace' }}
-                        />
+                        <div>
+                          <textarea
+                            className="form-input"
+                            value={p.template || ''}
+                            onChange={e => updateStagePrompt(stage.stage, pIdx, 'template', e.target.value)}
+                            rows={3}
+                            placeholder="Inline-Template mit {{placeholders}}..."
+                            style={{ width: '100%', fontSize: 12, textAlign: 'left', resize: 'vertical', fontFamily: 'monospace' }}
+                          />
+                          <div style={{ fontSize: 10, color: 'var(--text3)', marginTop: 4 }}>
+                            <div style={{ display: 'flex', flexWrap: 'wrap', gap: 4, marginTop: 4 }}>
+                              {['weight_data', 'nutrition_data', 'activity_data', 'sleep_data', 'vitals_baseline', 'blood_pressure', 'profile_id', 'today'].map(ph => (
+                                <code
+                                  key={ph}
+                                  onClick={() => {
+                                    const placeholder = `{{${ph}}}`
+                                    const currentValue = p.template || ''
+                                    updateStagePrompt(stage.stage, pIdx, 'template', currentValue + placeholder)
+                                  }}
+                                  style={{
+                                    padding: '2px 4px',
+                                    background: 'var(--surface2)',
+                                    borderRadius: 4,
+                                    fontSize: 9,
+                                    cursor: 'pointer',
+                                    border: '1px solid var(--border)'
+                                  }}
+                                  title="Klicken zum Einfügen"
+                                >
+                                  {'{{'}{ph}{'}}'}
+                                </code>
+                              ))}
+                            </div>
+                          </div>
+                        </div>
                       )}
                     </div>
                   </div>

frontend/src/pages/AdminPromptsPage.jsx

@@ -1,7 +1,7 @@
 import { useState, useEffect } from 'react'
 import { api } from '../utils/api'
 import UnifiedPromptModal from '../components/UnifiedPromptModal'
-import { Star, Trash2, Edit, Copy, Filter } from 'lucide-react'
+import { Star, Trash2, Edit, Copy, Filter, ArrowDownToLine } from 'lucide-react'
 
 /**
  * Admin Prompts Page - Unified System (Issue #28 Phase 3)
@@ -94,6 +94,49 @@ export default function AdminPromptsPage() {
     }
   }
 
+  const handleConvertToBase = async (prompt) => {
+    // Convert a 1-stage pipeline to a base prompt
+    if (prompt.type !== 'pipeline') {
+      alert('Nur Pipeline-Prompts können konvertiert werden')
+      return
+    }
+
+    const stages = typeof prompt.stages === 'string'
+      ? JSON.parse(prompt.stages)
+      : prompt.stages
+
+    if (!stages || stages.length !== 1) {
+      alert('Nur 1-stage Pipeline-Prompts können zu Basis-Prompts konvertiert werden')
+      return
+    }
+
+    const stage1 = stages[0]
+    if (!stage1.prompts || stage1.prompts.length !== 1) {
+      alert('Stage muss genau einen Prompt haben')
+      return
+    }
+
+    const firstPrompt = stage1.prompts[0]
+    if (firstPrompt.source !== 'inline' || !firstPrompt.template) {
+      alert('Nur inline Templates können konvertiert werden')
+      return
+    }
+
+    if (!confirm(`"${prompt.name}" zu Basis-Prompt konvertieren?`)) return
+
+    try {
+      await api.updateUnifiedPrompt(prompt.id, {
+        type: 'base',
+        template: firstPrompt.template,
+        output_format: firstPrompt.output_format || 'text',
+        stages: null
+      })
+      await loadPrompts()
+    } catch (e) {
+      alert('Fehler: ' + e.message)
+    }
+  }
+
   const handleSave = async () => {
     setEditingPrompt(null)
     setShowNewPrompt(false)
@@ -388,6 +431,21 @@ export default function AdminPromptsPage() {
                         >
                           <Edit size={16} color="var(--accent)" />
                         </button>
+                        {/* Show convert button for 1-stage pipelines */}
+                        {prompt.type === 'pipeline' && getStageCount(prompt) === 1 && (
+                          <button
+                            onClick={() => handleConvertToBase(prompt)}
+                            style={{
+                              background: 'none',
+                              border: 'none',
+                              cursor: 'pointer',
+                              padding: 4
+                            }}
+                            title="Zu Basis-Prompt konvertieren"
+                          >
+                            <ArrowDownToLine size={16} color="#6366f1" />
+                          </button>
+                        )}
                         <button
                           onClick={() => handleDuplicate(prompt)}
                           style={{