Include natshelper stage prefixes in nametag CSV export

[coder13]

Motivation

• Ensure nametag CSV exports include stage-aware prefixes when a WCIF uses the natshelper extension so multi-stage assignments are exported correctly.

Description

• Add a helper stagePrefixForNametags in src/pages/Competition/Export/index.tsx that derives a stage prefix from natshelper stage metadata and falls back to the room initial when unavailable.
• Update competingAssignmentToText to use stagePrefixForNametags while leaving group-number formatting unchanged.
• Change is limited to src/pages/Competition/Export/index.tsx and does not alter CSV column layout.

Testing

• Ran yarn check:type which completed successfully.
• Running npm run check:type failed in the container due to missing/incorrect tsc binary resolution in the environment and is unrelated to the code changes.

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Codex Task