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}

backend/app/routers/setup.py

@@ -0,0 +1,43 @@
+import uuid
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select, func
+from pydantic import BaseModel
+from ..dependencies import get_db
+from ..models.user import User
+from ..services.auth import hash_password, create_token
+
+router = APIRouter(prefix="/api/setup", tags=["setup"])
+
+
+class SetupRequest(BaseModel):
+    username: str
+    password: str
+    email: str = ""
+
+
+@router.get("/status")
+async def setup_status(db: AsyncSession = Depends(get_db)):
+    result = await db.execute(select(func.count()).select_from(User))
+    count = result.scalar()
+    return {"needsSetup": count == 0}
+
+
+@router.post("")
+async def run_setup(body: SetupRequest, db: AsyncSession = Depends(get_db)):
+    result = await db.execute(select(func.count()).select_from(User))
+    if result.scalar() > 0:
+        raise HTTPException(status_code=400, detail="Setup already completed")
+
+    admin = User(
+        id=str(uuid.uuid4()),
+        username=body.username,
+        email=body.email,
+        password_hash=hash_password(body.password),
+        is_admin=True,
+    )
+    db.add(admin)
+    await db.flush()
+    admin.token = create_token(admin.id)
+    await db.commit()
+    return {"success": True}