[fix] Remove global CSS rules to avoid conflicts #303 Refactored CSS to prevent global styling conflicts with downstream projects. Fixes #303

[Unnati-Gupta24]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #303 .

Description of Changes

The initial issue was that CSS styles were being applied globally using the * selector, which can cause conflicts in larger projects. Here's how I fixed it:

Created a scoped container structure:

• Added .netjsongraph-container as main wrapper

• Added #graphChartContainer for the graph content

• This contains styles to just these elements instead of affecting entire page

• Fixed height/positioning issue:

• Set proper html/body height to 100%

• Made container absolute positioned with full dimensions
  This ensures graph fills available space correctly

Updated JavaScript:

• Changed graph initialization to target specific container
• This connects the new DOM structure to the graph rendering

This approach prevents style leaks while maintaining the original functionality.

@pandafy please review this pull request.
I have made appropriate changes to fix the issue.