feat(web): support 5x2 split workspace

[SuzuMiyaYuuGi]

Summary

• add a 5-column by 2-row split workspace with up to 10 panes
• persist bottom-row placement and keep drag/drop feedback aligned with the layout

This change is contributed by SuzuMiyaYuuGi (UG).

Verification

• focused split tests: 25 passed
• apps/web TypeScript check: no errors from this change
• git diff --check: passed

[coderabbitai]

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[Type-Delta]

This feature has been implemented separately from this idea. Closing this PR.