fix: align account settings tab backgrounds

[Rello]

Motivation

• QTabWidget pane and tab pages had inconsistent backgrounds causing visual mismatch in the account settings dialog.
• File Provider and Connection Settings content did not reliably stretch to fill the tab area, leading to unused space or clipped QML content.
• A QSizePolicy include was required to set the expanding policy for the embedded QML widget.

Description

• Updated the QTabWidget stylesheet to use palette(alternate-base) for the pane and for #standardSyncTab, #fileProviderTab, and #connectionSettingsTab.
• Enabled setAutoFillBackground(true) and set Qt::WA_StyledBackground on the three tab pages to ensure the styled backgrounds are applied.
• Set QVBoxLayout::setSpacing(0), applied QSizePolicy::Expanding to the file provider QML widget, and added the File Provider and Network Settings widgets with stretch 1 so they fill the available space.
• Added #include <QSizePolicy> to support the size policy usage.

Testing

• No automated tests were run.

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Codex Task

[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-38.zip

Digest: sha256:64691b92fbd205a9c38d8ef8fc58806727a7e302ee9b1c5234b5adecbace3475

To test this change/fix you can download the above artifact file, unzip it, and run it.

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