WIP - feat(measurements): add category grouping and dynamic formula support

[pankaj-basnet]

Summary

This PR adapts the Flutter client to two planned (ongoing) backend capabilities given below:

1. Category Grouping (Planned PR) — Measurement categories can now be linked to a MeasurementGroup via a foreign key on the backend. For example, a "Blood Pressure" group can contain both a "Systolic" and a "Diastolic" category, letting the app surface them together.
2. Dynamic / Formula-calculated Categories (Ongoing PR) — The /api/v2/measurement/ action endpoint returns auto-computed entries (e.g., BMI derived from body weight + height) as well as the current user's created ones. The Flutter client now routes requests for such categories to this endpoint and enforces a read-only UI so users cannot accidentally add or edit auto-computed values.

What changed

Layer	File	Change
Model	measurement_group.dart	New model for measurement groups
Model	measurement_category.dart	Added groupId, groupDetail, formula, and dynamicParams
Provider	measurement.dart	Group CRUD, updated editCategory signature
Screen	measurement_categories_screen.dart	Group filter chips
Screen	measurement_entries_screen.dart	Read-only scaffold for dynamic categories
Widget	categories.dart	Group-aware filtering in list
Widget	categories_card.dart	Group chip + auto-calculated badge
Widget	entries.dart	Hide edit/delete options for dynamic entries
Widget	forms.dart	Group + formula dropdowns in category form
Mock data	mock_measurement_data.dart	Rich dummy data for UI development

Related Issue(s)

• wger#2338
• wger#1560
• wger#1723

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Set a 100-character limit to avoid white space diffs (run dart format .)

UI Screenshots

[rolandgeider]

FYI, since we're getting close to finishing a big release (see e.g. #1183), I'm moving this to the next version 2.7 since the dynamic measurements come with some changes to the backend and frontend and I'd be more comfortable with some time