Base block: background image position/scale controls

The base block's image mode only offers Cover and Contain. Graphic blocks have alignment (horizontal/vertical) which controls background-position, but the base block doesn't expose these.

What to add

• Background position controls (center, top, bottom, left, right, or x/y percentage)
• Possibly background-size with custom values (not just cover/contain)

Where

• Frontend: BaseConfiguration.vue uses BackgroundStyleSection which already supports @update:settings. The alignment controls exist in FlexAlignmentSection but aren't wired up for base block's image mode.
• Engine: getBackgroundProperties() in helpers.ts already reads horizontalAlignment and verticalAlignment from the config and applies them as background-position. The data just needs to flow from the frontend.

Priority

Low -- cover/contain handles most cases. Nice-to-have for full-bleed background images that need precise positioning.