feat(spx-gui): raise sb upload limit to 128 MiB

[fennoai]

Summary

Raise the sb (.sb2/.sb3) upload limit from 64 MiB to 128 MiB on the frontend, tracking #3351.

Changes

• spx-gui/src/apis/project-conversion.ts: change the hardcoded local pre-check maxFileSize from 64 * 1024 * 1024 to 128 * 1024 * 1024. This was the only hardcoded 64 MiB frontend limit.
• docs/openapi.yaml: update the /project-conversions request body description from "Maximum size is 64 MiB." to "Maximum size is 128 MiB.".

Notes

• The general cloud upload path (spx-gui/src/models/common/cloud.ts) already follows the backend-provided maxSize and needs no change — it will pick up 128 MiB once the backend raises the value.
• The UpInfo.maxSize example in docs/openapi.yaml (26214400) is a generic illustrative value unrelated to the sb limit, so it is left unchanged.
• Requires the companion backend change in goplus/builder-backend (maxMultipartMemory 64 → 128 MiB) to fully take effect.

Fixes #3351

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[fennoai]

Review summary

Small, focused change that raises the frontend sb (.sb2/.sb3) upload pre-check limit from 64 MiB to 128 MiB and updates the matching OpenAPI description. The change is correct and internally consistent:

• project-conversion.ts is the only hardcoded frontend limit (confirmed via repo-wide search for 64 * 1024 * 1024 / 67108864 / 64 MiB — no other references remain).
• The general cloud upload path in models/common/cloud.ts already follows the backend-provided maxSize, so it needs no change here.

One minor, non-blocking nit inline. As the PR description notes, the companion backend change (maxMultipartMemory 64 → 128 MiB) is required for this to fully take effect; this frontend-only pre-check will otherwise reject at 128 MiB while the backend still rejects earlier — worth landing them together.

No blocking issues found.