Populate the wizard remote-folder picker from stored credentials (#147)

[gnacho]

Closes #147.

Problem

During the initial account setup wizard, the dropdown for selecting server folders does not expand and fails to load the list of remote folders. Environment: CachyOS/Arch, Nextcloud, browser-login auth.

Root cause (verified, red-green)

populate_wizard_remote_picker (setup.rs) read the password from ctx.widgets.password_entry.text(), but after login (browser flow in browser_flow_succeeded, or manual in manual_login) the wizard clears the field and stores the secret in the keyring. With an empty field, the if password.is_empty() { return None; } short-circuited before calling list_remote_folders, so the GtkDropDown opened with an empty GtkStringList - "does not expand nor list". list_remote_folders itself was fine; the defect was the credential source. (The Settings picker already resolves from the keyring; the wizard one did not.)

Change

src/ui/setup.rs:

• wizard_remote_picker_password(server, username): resolves from the credential store (get_for_account).
• fetch_wizard_remote_folders(api, server, username): synchronous and injectable.
• populate_wizard_remote_picker uses it via NextcloudApi::new().

3 new tests, including the reproduction wizard_picker_lists_remote_folders_from_stored_credentials (keyring seeded like CredentialsStore::set + fake WebDAV): red with the old empty-field behavior, green with the fix.

Caveat

The visual popup expansion in a real GTK session (server + browser login) is not reproducible headless; the fix is verified at the picker-logic level, which is exactly what the wizard runs.

Gate: build OK, clippy clean (-D warnings), fmt OK, 694 tests.