Add first-run setup wizard and file browser for library creation

- On first start (no users in DB), show a setup page in the web UI
  to create the admin account instead of reading credentials from .env
- New public endpoints: GET /api/setup/status, POST /api/setup
- New admin endpoint: GET /api/filebrowser?path=... for directory listing
- FileBrowser modal component with navigation, used in Admin > Libraries
- Remove _seed_admin / _seed_default_library from server startup

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

frontend/src/components/common/FileBrowser.tsx

@@ -0,0 +1,103 @@
+import React, { useState, useEffect } from 'react'
+import { Folder, ChevronRight, ChevronUp, X, Check, Loader2 } from 'lucide-react'
+import api from '../../api/client'
+
+interface Entry { name: string; path: string; isDir: boolean }
+
+interface Props {
+  initialPath?: string
+  onSelect: (path: string) => void
+  onClose: () => void
+}
+
+export default function FileBrowser({ initialPath = '/', onSelect, onClose }: Props) {
+  const [path, setPath] = useState(initialPath)
+  const [entries, setEntries] = useState<Entry[]>([])
+  const [parent, setParent] = useState<string | null>(null)
+  const [loading, setLoading] = useState(false)
+  const [error, setError] = useState('')
+
+  const load = async (p: string) => {
+    setLoading(true)
+    setError('')
+    try {
+      const r = await api.get('/api/filebrowser', { params: { path: p } })
+      setPath(r.data.path)
+      setParent(r.data.parent)
+      setEntries(r.data.entries.filter((e: Entry) => e.isDir))
+    } catch (err: any) {
+      setError(err.response?.data?.detail || 'Fehler beim Laden')
+    } finally {
+      setLoading(false)
+    }
+  }
+
+  useEffect(() => { load(initialPath) }, [])
+
+  return (
+    <div className="fixed inset-0 bg-black/70 z-50 flex items-center justify-center p-4" onClick={onClose}>
+      <div
+        className="bg-surface border border-white/10 rounded-xl w-full max-w-lg shadow-2xl flex flex-col"
+        style={{ maxHeight: '80vh' }}
+        onClick={(e) => e.stopPropagation()}
+      >
+        {/* Header */}
+        <div className="flex items-center justify-between px-4 py-3 border-b border-white/10 flex-shrink-0">
+          <h3 className="text-sm font-semibold text-white">Ordner auswählen</h3>
+          <button onClick={onClose} className="text-gray-400 hover:text-white">
+            <X size={16} />
+          </button>
+        </div>
+
+        {/* Current path */}
+        <div className="px-4 py-2 bg-white/5 border-b border-white/10 flex items-center gap-2 flex-shrink-0">
+          {parent && (
+            <button onClick={() => load(parent)} className="text-gray-400 hover:text-white flex-shrink-0">
+              <ChevronUp size={16} />
+            </button>
+          )}
+          <p className="text-xs text-gray-300 font-mono truncate">{path}</p>
+        </div>
+
+        {/* Entry list */}
+        <div className="overflow-y-auto flex-1">
+          {loading ? (
+            <div className="flex justify-center py-10">
+              <Loader2 size={24} className="text-primary animate-spin" />
+            </div>
+          ) : error ? (
+            <p className="text-red-400 text-sm px-4 py-6">{error}</p>
+          ) : entries.length === 0 ? (
+            <p className="text-gray-500 text-sm px-4 py-6">Keine Unterordner</p>
+          ) : (
+            entries.map((e) => (
+              <button
+                key={e.path}
+                onClick={() => load(e.path)}
+                className="w-full flex items-center gap-3 px-4 py-2.5 hover:bg-white/5 text-gray-300 hover:text-white text-sm text-left transition-colors"
+              >
+                <Folder size={16} className="text-yellow-500 flex-shrink-0" />
+                <span className="flex-1 truncate">{e.name}</span>
+                <ChevronRight size={14} className="flex-shrink-0 text-gray-600" />
+              </button>
+            ))
+          )}
+        </div>
+
+        {/* Footer */}
+        <div className="flex items-center justify-between px-4 py-3 border-t border-white/10 flex-shrink-0">
+          <button onClick={onClose} className="text-gray-400 text-sm hover:text-white">
+            Abbrechen
+          </button>
+          <button
+            onClick={() => { onSelect(path); onClose() }}
+            className="flex items-center gap-2 bg-primary text-black px-4 py-2 rounded-lg text-sm font-medium hover:bg-primary/80"
+          >
+            <Check size={14} />
+            Auswählen
+          </button>
+        </div>
+      </div>
+    </div>
+  )
+}