Dimensions tab persistence #839

Part of #836. Wires the Dimensions tab to the existing endpoint. Depends on the Color/foundation PR (lifts modal-level state lifting and save UX into place).

Scope

Lift state for the four Dimensions panels to modal level and include their values in the payload sent by handleUpdateClick:

• LayoutPanel → settings.layout.*
• SpacingSettingsPanel → settings.spacing.* (flag fields)
• SpacingPresetsPanel → settings.spacing.spacingSizes
• SpacingUnitsPanel → settings.spacing.units

Acceptance criteria

• Each Dimensions sub-panel's edits persist on Update.
• Loading / success / error UX matches Color tab.
• No regression to Color persistence.

Out of scope

• Other tabs.

References

• Tracking issue: [Tracking] Edit Theme Settings: phased implementation #836