feat: add progress steps indicator (Upload → Configure → Export → Download)

[arnavgoel17]

Summary

• Adds a 4-step workflow stepper at the top of the editor: Upload → Configure → Export → Download.
• Step state derives from the editor's file/status (per issue mapping): Upload when no file, Configure when a file is loaded and idle, Export during loading-engine/exporting, Download when done.
• Completed steps show a checkmark; the active step is highlighted with the accent color; connectors fill as steps complete.
• Accessible: wrapped in a labelled nav + ol, each step uses aria-current="step" when active, and a visually-hidden state label is provided for screen readers. Respects prefers-reduced-motion.

Acceptance Criteria

• 4-step indicator visible at top of page
• Active step highlighted
• Completed steps show checkmark
• Accessible with ARIA

Test plan

• npx tsc --noEmit — passes
• npm run lint — passes
• npm run build — builds successfully

Fixes #98

🤖 Generated with Claude Code

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[github-actions]

👋 This PR has been inactive for 21 days. It will be automatically closed in 14 days if there's no further activity.

If you need help with this PR, please comment or ping a maintainer.
Make sure your branch is rebased onto main and all CI checks pass.