Refactor html report to use robust sitegraph template

[saurabhsharma2u]

This PR addresses a code health issue in plugins/core/src/report/html.ts by consolidating the graph visualization logic.

Previously, generateHtml contained a simplified, inline D3 implementation that duplicated logic found in plugins/core/src/report/sitegraph_template.ts. This PR refactors generateHtml to use SITEGRAPH_HTML from the template file, ensuring a single, robust implementation is used across the codebase.

Key changes:

• generateHtml now injects graphData and metrics into SITEGRAPH_HTML.
• The html property is removed from nodes in the injected data to keep the report lightweight, similar to how the CLI export handles it.
• Tests in plugins/core/tests/html_report.test.ts are updated to verify that the template and data are correctly injected.
• Frontend verification was performed, confirming the new report renders correctly.

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PR created automatically by Jules for task 4622851668163800598 started by @saurabhsharma2u

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