Add sample fixtures and the 'Load sample project' flow

[honor2030]

Goal

Let users try the tool without finding or creating their own test files.

Acceptance criteria

• Sample projects load and reach the review table in one click.
• Sample data is realistic enough to show common matching patterns.
• The sample flow works identically to a user-uploaded project.

Task list

• Create fixture files (old + new URL sets in CSV and XML).
• Add 'Load sample project' button to the upload screen.
• Wire button to skip file picker and pre-populate the store.
• Verify the full pipeline runs on sample data.

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Estimate: 0.5 day