fix: incorrect name for background size on Flip Block php css

[Soare-Robert-Daniel]

Closes https://github.com/Codeinwp/otter-internals/issues/187

Summary

Fix incorrect name for variable identification for image background size on Flip Block PHP CSS.

Screenshots

Editor

Front/User

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Test instructions

1. Insert a Flip Block
2. Add small images as background for the front and back sides
3. Check if the settings are applied.

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Checklist before the final review

• Included E2E or unit tests for the changes in this PR.
• Visual elements are not affected by independent changes.
• It is at least compatible with the minimum WordPress version.
• It loads additional script in frontend only if it is required.
• Does not impact the Core Web Vitals.
• In case of deprecation, old blocks are safely migrated.
• It is usable in Widgets and FSE.
• Copy/Paste is working if the attributes are modified.
• PR is following the best practices

[pirate-bot]

Bundle Size Diff

Package	Old Size	New Size	Diff
Animations	264.29 KB	264.29 KB	0 B (0.00%)
Blocks	1.52 MB	1.52 MB	0 B (0.00%)
CSS	93.54 KB	93.54 KB	0 B (0.00%)
Dashboard	196.3 KB	196.3 KB	0 B (0.00%)
Onboarding	153.95 KB	153.95 KB	0 B (0.00%)
Export Import	91.01 KB	91.01 KB	0 B (0.00%)
Pro	362.27 KB	362.27 KB	0 B (0.00%)

[pirate-bot]

E2E Tests

Playwright Test Status:

Performance Results

serverResponse: 247.3, firstPaint: 175.35, domContentLoaded: 1604.9, loaded: 1605.9, firstContentfulPaint: 3566.65, firstBlock: 6945.8, type: 12.82, minType: 12, maxType: 14.36, typeContainer: 9.76, minTypeContainer: 7.69, maxTypeContainer: 12.34, focus: 35.2, minFocus: 28.78, maxFocus: 45.22, inserterOpen: 24.44, minInserterOpen: 21.48, maxInserterOpen: 27.98, inserterSearch: 0.78, minInserterSearch: 0.64, maxInserterSearch: 1.08, inserterHover: 3.2, minInserterHover: 2.66, maxInserterHover: 4.26, listViewOpen: 150.33, minListViewOpen: 137.71, maxListViewOpen: 160.67

[pirate-bot]

🎉 This PR is included in version 3.0.0 🎉

The release is available on GitHub release

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