CONV-082: Add format step placeholder

[menvil]

Show stepper and format placeholder after successful upload.

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Summary by cubic

Adds a conversion stepper and shows a format selection placeholder after a successful upload to guide users to the next step. Aligns with Linear CONV-082.

• New Features
  • Added stepper with steps: File → Format → Settings → Convert; active is File during upload, Format after upload.
  • After upload, switch to the Format step and render the placeholder (“Choose output format”; “Target format selection will be added in Phase 7”); added a feature test for this flow.

^{Written for commit 6a53bd0. Summary will update on new commits.}

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/CONV-082-add-format-step-placeholder

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