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>

backend/app/routers/filebrowser.py

@@ -0,0 +1,37 @@
+import os
+from fastapi import APIRouter, Depends, HTTPException, Query
+from ..dependencies import require_admin
+from ..models.user import User
+
+router = APIRouter(prefix="/api/filebrowser", tags=["filebrowser"])
+
+
+@router.get("")
+async def browse(
+    path: str = Query("/"),
+    _admin: User = Depends(require_admin),
+):
+    path = os.path.normpath(path)
+    if not os.path.isdir(path):
+        raise HTTPException(status_code=404, detail="Pfad nicht gefunden")
+
+    try:
+        names = sorted(os.listdir(path), key=lambda n: n.lower())
+    except PermissionError:
+        raise HTTPException(status_code=403, detail="Zugriff verweigert")
+
+    entries = []
+    for name in names:
+        if name.startswith("."):
+            continue
+        full = os.path.join(path, name)
+        try:
+            is_dir = os.path.isdir(full)
+        except OSError:
+            continue
+        entries.append({"name": name, "path": full, "isDir": is_dir})
+
+    parent_path = os.path.dirname(path)
+    parent = parent_path if parent_path != path else None
+
+    return {"path": path, "parent": parent, "entries": entries}