CLI account setup only supports one sync folder per invocation

## Problem

The command-line account setup feature (`--userid`, `--apppassword`, `--serverurl`, `--localdirpath`, `--remotedirpath`) currently has two limitations that make it impossible to configure more than one sync folder:

### 1. Repeated `--localdirpath` / `--remotedirpath` arguments are silently ignored

In `src/gui/accountsetupcommandlinemanager.cpp`, each argument is stored in a single `QString` member variable (`_localDirPath`, `_remoteDirPath`). Passing the same flag multiple times simply overwrites the previous value — only the last pair survives into the setup job.

### 2. A second invocation is rejected because the account already exists

`src/gui/accountsetupfromcommandlinejob.cpp` checks whether the account (`userid@hostname`) is already registered and exits with code 1 if so. This means there is no way to add a second sync folder to an already-configured account via the CLI — not in a single invocation, and not by running the app a second time.

## Expected behaviour

It should be possible to configure multiple sync folders for the same account in one CLI invocation, e.g.:

```
nextcloud \
--userid alice \
--apppassword secret \
--serverurl https://cloud.example.com \
--localdirpath ~/Documents --remotedirpath /Documents \
--localdirpath ~/Photos --remotedirpath /Photos
```

## Suggested fix

1. Change `_localDirPath` and `_remoteDirPath` in `AccountSetupCommandLineManager` from `QString` to `QList`, accumulating each flag value instead of overwriting.
2. In `AccountSetupFromCommandLineJob`, when the account already exists, skip account creation and proceed directly to folder setup instead of aborting.
3. Iterate over all requested folder pairs and call `setupLocalSyncFolder` for each one.

A working implementation is available as a pull request.

## Affected files

- `src/gui/accountsetupcommandlinemanager.h` / `.cpp`
- `src/gui/accountsetupfromcommandlinejob.h` / `.cpp`

Start with src/gui/accountsetupcommandlinemanager.h/.cpp and src/gui/accountsetupfromcommandlinejob.h/.cpp to trace how folder arguments are collected and how existing accounts are handled. Confirm the command accepts multiple local/remote folder pairs, supports adding folders to an existing account, and completes setup for every pair.

Written by the indexing model from the issue text.