Modernize applet UI

Bharadwaj Rajurequested to merge
work/bharadwaj/improve-plasmoid-ui into master

Changes the applet view to use expandable list items like other Plasma applets (c.f. Plasma Vault, for one), adds a standard placeholder message item for the no-plans case, and changes the capitalization of some button labels to be in line with KDE HIG (e.g. "Save backup" to "Save Backup"), per https://develop.kde.org/hig/text_and_labels/

Screenshot_20260722_204157

This is part of kde-linux/kde-linux#666

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