fix: notify user when export falls back to WebM

[Harshith1702]

Fixes #30

MP4 fails silently and exports as WebM with zero indication to the user.
This fixes that.

Changes:

• types.ts: added usedFallback and fallbackReason as optional fields on ExportResult
• ffmpeg.ts: compare recipe.format vs actual output format after export, tag result if they differ
• DownloadResult.tsx: render a warning banner when usedFallback is true

VideoEditor.tsx untouched, already passes the full result object down.

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👋 Thanks for your PR, @Harshith1702!

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[Harshith1702]

Hey @magic-peach it's been 2 weeks...., any review