Migrate UI layer to React while preserving spreadsheet core

[code-by-sia]

Summary

Migrate only the OpenWebSheet UI layer to React. The spreadsheet core, rendering/data model, formulas, import/export logic, and file format handling should remain separate from the React migration unless a small adapter is required.

Scope

• Rebuild UI shell, menus, dialogs, toolbars, panels, and view controls using React.
• Create clear boundaries between React UI components and the existing spreadsheet core APIs.
• Add adapters/hooks where needed for UI-to-core communication.
• Preserve existing canvas/spreadsheet behavior during the migration.
• Keep the core framework-agnostic so it can be tested and evolved independently.

Out of scope

• Rewriting the spreadsheet engine/core logic.
• Rewriting formula evaluation.
• Rewriting import/export functionality.
• Changing the file format or data model unless required for UI integration.

Suggested acceptance criteria

• React app shell can host the existing spreadsheet surface.
• Main UI controls are implemented as React components.
• Existing spreadsheet core behavior remains unchanged.
• UI-to-core boundaries are documented.
• Regression tests cover key UI workflows and confirm no core behavior regressions.

Notes

This should be treated as a UI migration issue, not a full application rewrite.