[tristan0x]

Fixes #753

On the first synchronization of a cloud project, clicking "..." on the local directory page opened the folder picker, and cancelling it immediately opened a second one.

Changes

• Connect the local directory button once. The clicked connection was made in show_project_local_dir_selection(), which runs on every showEvent. The dialog is shown twice, by show() in show_transfer_dialog() and then by open() in show_sync_popup(), so the button ended up connected twice. The connection now lives in the constructor.
• Keep the current directory on cancel. select_folder() no longer overwrites the line edit with an empty string when the picker is cancelled. The picker also gets a parent window and a "Select directory" caption instead of Qt's generic "Find Directory". This also applies to the QField directory picker in the synchronize dialog.

How to test

1. Open a cloud project that has never been synchronized locally and start the synchronization.
2. On the local directory page, click "..." and then Cancel: no second picker appears and the pre-filled path is kept.
3. Click "..." again and pick a folder: the field is updated.
4. In "Synchronize from QField", check that the QField directory picker behaves the same way.

Out of scope

The issue also mentions that typing a directory name does not offer to create it. That comes from Qt's native QFileDialog, which only accepts existing directories. The typed directory is already created with os.makedirs when the synchronization starts.