IA-956: feat: Add subject unassignment and improve subject/assignment loading reliability

Summary

Adds the ability to unassign subjects from a child, and improves the robustness of subject fetching and assignment state management.

Changes

ChildSubjectsManager

• Added unassign logic in saveChanges: now computes toAssign and toUnassign diffs against currently assigned subjects, and calls unassignSubjectFromChild in parallel for removed subjects
• Fixed toggleSubject to use local currentSelected count from modalSubjects instead of stale selectedCount from outer scope
• Added ListEmptyComponent in the modal FlatList to handle loading and empty states gracefully
• Fixed early-return guard to check child.uuid instead of child.id
• Combined assignedSubjectsLoading and subjectsLoading into a single loading guard block
• Modal subtitle now simplified to per-child quota wording

subscriptionStore

• Added unassignSubjectFromChild(childUuid, subjectUuid) action calling DELETE .../child/:childUuid/subject/:subjectUuid
• fetchAssignedSubjectsByChild now resets assignedSubjects: [] before fetching to avoid stale data flash

subjectStore

• Added guard for empty levelUuid to skip fetch early
• Resets subjects: [] before each fetch
• Adds pagination params { size: 1000, page: 0 } to the request
• Handles both array and paginated ({ content: [] }) response shapes
• Normalizes subjectUuid by falling back to s.uuid if missing

subscriptionEndpoints

• Added unassignSubject(childUuid, subjectUuid) endpoint:
  DELETE .../assign-subjects/child/:childUuid/subject/:subjectUuid