Extend "Extra Field" capability to allow dynamic/calculated "Formula" Extra Fields

[akira69]

A user may want to have a dynamic "extra field" that is based on mathematical / text functions of existing fields

• e.g. "time since last dried", "time since last used", concatenated text, rounded numbers, etc.

This concept would add a "Formula" extra field capability via JSON editor within Filaman UI (by plugin most likely - or under/next to existing extra fields entry points)

• see wip(settings): formula fields JSON-first base and preview akira69/Spoolman_Labels#14 for inspiration
• this feature would directly address and directly provide features requested by the larger spoolman community
  • Implementation into Filaman would provide a edge/benefit above Spoolman
• The user can see all available fields and available functions to operate on those fields by the interface.
• Results:
  Entry table

JSON interface with operators/helpers and field name variables listed

A phase 2 could extend simplicity for users via a formula helper that provides guided coding/variable picking with a content type-aware interface

• see wip(settings): formula field guided builder follow-up akira69/Spoolman_Labels#13 for inspiration

[akira69]

Status update: Working implementation is in akira69#21. It adds a "Formula" extra field type alongside existing field types — formula expressions are evaluated at render time using field variables from the spool/filament/manufacturer context. The JSON-first editor with variable picker from the concept above is reflected in the PR's UI. Pending upstream review.