fix: size sync folders panel to contents

[Rello]

Motivation

• The sync folders listing consumed a large fixed vertical space even when empty, leading to a big empty box in the settings dialog.
• The folder listing should behave like the other account panels and only request the space it needs and be movable into its own panel for future layout changes.

Description

• Move the sync folders UI out of the tab and into a dedicated syncFoldersPanel in src/gui/accountsettings.ui so it sits alongside the account status and file provider panels.
• Make the tree view report a dynamic preferred height by adding FolderStatusView::sizeHint() in src/gui/folderstatusview.{h,cpp} which computes height from the number of rows, per-row height and scrollbar presence.
• Enable size-to-contents for the view via _ui->_folderList->setSizeAdjustPolicy(QAbstractScrollArea::AdjustToContents) and refresh geometry when the model changes by connecting rowsInserted, rowsRemoved and modelReset to updateGeometry in src/gui/accountsettings.cpp.
• Update stylesheets to target the new syncFoldersPanelContents widget and adjust the grid row stretch in src/gui/accountsettings.ui so the layout grows/shrinks correctly.

Testing

• No automated tests were run as part of this change.

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Codex Task