fix(css): stabilize hover system regex replacements and prevent duplicate transition injection

[ANSHIKATYAGI30]

What does this PR do?

Changes made-:

• Fixed incorrect conditional logic that prevented grouped hover styles from being appended properly.
• Improved regex replacement safety for hover animation transformations and transitions.
• Prevented duplicate CSS transition injection during repeated script runs.
• Refactored hover-style injection logic for better maintainability and deterministic behavior.
• Added safer detection checks before appending generated CSS blocks.
• Improved compatibility of CSS modernization script with existing style structures.

Related Issue

Closes #731

Checklist

• I have tested my changes
• My code follows project guidelines
• No new errors introduced

[github-actions]

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[ANSHIKATYAGI30]

@neeru24
I checked the failing test and the issue appears to come from tests/test_app.py rather than the changes in modernize_hover.py.

Current API response:
"File content does not match any supported image format."

But the test still expects:
"Invalid image"

The failure occurs in:
tests/test_app.py::test_post_api_analyze_invalid_image

Since my PR only modifies hover animation/UI logic in app.py, I did not change the image validation flow.
Please review the PR whenever convenient.
Thank you!