fix(export): size a GIF from the source, not the hidden MP4 tier

[EtienneLescot]

Summary

A GIF's size no longer depends on the MP4 quality tier, a control the GIF settings hide. It starts from the "Source" size (the smallest clip's true footprint, so nothing is upscaled), then the size preset caps its height.

• Applies to both the export call and the destination summary, through one gifOutputDims.
• Original now means the source size.
• MP4 sizing is unchanged.

Reported on #814 with a 640×360 source: README GIF gave 852×480 when picked directly, 640×360 after Studio. It is now 640×360 both ways.

Related issue

Refs #814

Type of change

• Bug fix

Release impact

• Patch

Desktop impact

• Not platform-specific

Testing

• Regression tests in ExportDialog.params.test.tsx: the tester's path (summary and export params, direct vs. after Studio), and every MP4 tier × GIF size preset on a 640×360 source. Both failed before the fix with 852×480.
• npx vitest --run src/components/ai-edition/: 69/69 files, 549 tests pass.
• npx tsc --noEmit and npx tsc -p tsconfig.test.json --noEmit: clean.
• npm run lint: the only errors are format errors on files checked out as CRLF (core.autocrlf), none in this diff. biome check on the two changed files passes.

Left as is

The CLI (src/cli/CliExportRunner.tsx) still sizes a GIF from --quality. That is documented in website/docs/cli.md ("A GIF starts from this size too"), so aligning it would change a documented flag and its translations.

🤖 Generated with Claude Code

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