[AAE-17909][AAE-17964] fix silent-refresh url is not set with the value - fix background image set to undefined

[alep85]

… app.config.json

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation
• Other... Please describe:

What is the current behaviour? (You can also link to an open issue here)

• https://alfresco.atlassian.net/browse/AAE-17909

• https://alfresco.atlassian.net/browse/AAE-17964

What is the new behaviour?

• The silent-refresh url is set with the value from app.config.json
• Set default value to header backgroundImage

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[sonarqubecloud]

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