feat: add skeleton loader for document generation preview (#819)

[Dharshin1]

Description

This PR improves the loading experience on the Document Generate page by replacing the waiting spinner with a skeleton loader while the legal document is being generated.

Issue Resolved

Closes #819

Changes Made

• Added a skeleton loading UI for document generation.
• Updated the document generation flow to navigate to the preview state immediately while the API request is in progress.
• Display skeleton placeholders until the generated document content is received.
• Preserved existing document preview and download functionality.

Testing

• Frontend tests pass successfully.
• Existing document generation functionality remains intact.
• Loading state displays a skeleton placeholder instead of only a spinner.

Test Results

Test Files  5 passed (5)
Tests      16 passed (16)

Type of Change

Feature Enhancement

Notes

This change improves perceived performance and provides users with a more informative loading experience while waiting for legal document generation.

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