Add formula fields to collections

[priethor]

What

Closes #108.

Adds formula fields to collections. A formula is read-only in the row, but can be configured from the field menu with field("Name") or prop("Name"), arithmetic, text helpers, comparisons, if(), and a small set of date helpers, including now(). The computed value shows up in table rows, row detail, sorting, filtering, and indexed field reads when the result type supports it.

Why

Computed fields are part of the core collection workflow. They let a table express values that are derived from the row itself, like totals, labels, checks, and date offsets, without turning those values into another thing the user has to edit.

How

• Evaluating formulas on the server so every row surface reads the same value.
• Storing formula output in the normal row meta while blocking direct edits.
• Resolving field references to stable field IDs so renames keep working.
• Rejecting invalid formulas, self-references, cycles, mixed if() result types, and unsupported multi-value references before save.
• Treating now() formulas as snapshot values that refresh when formulas are evaluated, not as live ticking values.
• Keeping v0 scoped to same-row, single-value formulas. Multi-select, relation, rollup, regex, and collection-level operations are left for follow-ups.
• Adding the formula editor, syntax highlighting, field/function autocomplete, and the formula language reference.

Testing Instructions

1. Create a collection with number fields named Price and Tax.
2. Add a formula field named Total with field("Price") + field("Tax").
3. Confirm the row shows the computed total.
4. Edit Price or Tax and confirm Total updates.
5. Enter an invalid formula and confirm the inline error appears without breaking the row view.
6. Sort and filter by Total.
7. Open the formula reference panel and try autocomplete for fields and functions.

Screen recording

Grabacion.de.pantalla.2026-06-03.a.las.19.27.12.mov