Implement advanced formula and expression features

[code-by-sia]

Summary

The roadmap lists formula and expressions as incomplete, with only Basic Support checked and Advanced features still missing.

Scope

Add advanced spreadsheet formula/expression support beyond the current basic implementation.

Suggested acceptance criteria

• Support a broader set of spreadsheet functions and expression operators.
• Handle nested formulas and cell/range references reliably.
• Report formula parse/evaluation errors clearly in the UI.
• Add unit tests for advanced formula parsing and evaluation behavior.
• Document the supported formula syntax and limitations.

Source

Tracked from the README roadmap:

* [ ] formula and expressions
  * [x] Basic Support
  * [ ] Advanced features