fix: refine settings layout spacing

[Rello]

Motivation

• Prevent small vertical jumps when switching navigation items by fixing unexpected margins/paddings around settings content.
• Make section headings like General settings, Advanced and Info sit inside their panels and use regular weight so they visually align with their group boxes.
• Remove the darker wrapper background and extra paddings around the stacked panels and align the account connection status with the storage usage controls.

Description

• Moved the General settings, Advanced and Info heading labels into their respective QGroupBox widgets in src/gui/generalsettings.ui and removed the bold font usage so the headings are non-bold and part of the panels.
• Removed outer paddings by setting the main grid layouts' margins and spacing in src/gui/generalsettings.ui and src/gui/accountsettings.ui to remove unexpected offsets and added a small consistent spacing between panels.
• Aligned account connection and storage rows by inserting a fixed spacer (storageStatusSpacer) before the quota label in src/gui/accountsettings.ui so the connectLabel and quotaInfoLabel line up.
• Eliminated the darker background/padding around the content wrapper by clearing shellLayout margins, adding settings_content object name to the content container, and making #settings_content transparent in src/gui/settingsdialog.cpp so the stacked widget area is visually flush.

Testing

• No automated tests were run on these UI/layout changes.

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Codex Task