IA-958 : feat: Rework fetchAssignedSubjects to use bulk UUID lookup with client-side filtering and category label normalization

Summary

Replaces the single-endpoint assigned subjects fetch with a two-step approach: first resolve assigned UUIDs, then fetch full subject details in bulk. Adds French category label mapping and fixes client-side filtering. Also cleans up the Learning screen's fetch effect.

Changes

subjectEndpoints

• Added getSubjectsByUuids endpoint: POST .../subjects/by-uuids

subjectStore (mobile)

• Added categoryLabels map to normalize API category keys (e.g. MATHEMATIQUES) into French display labels
• Rewrote fetchAssignedSubjects with a two-step flow:
  1. GET assigned subjects to extract subjectUuid list
  2. POST to getSubjectsByUuids with { uuids, childUuid } to fetch full subject details
• Applied categoryLabels mapping during subject normalization, with fallback to raw value
• Normalizes subjectUuid by falling back to s.uuid
• Moved category and name filtering to client-side on the fetched results, replacing the previous query-param approach
• Added early return with empty subjects when no UUIDs are assigned

Learning screen

• Simplified fetch useEffect: removed the null check branch and unified category argument — passes undefined for "Tous" instead of undefined vs filtered array
• Omitted fetchAssignedSubjects from the dependency array intentionally to prevent infinite re-fetch loop (documented in comment)
• Passes searchQuery || undefined to avoid sending empty string to the store