Add Blindeforbundet assignment count badge to contacts list

Extend the AssignedContact model from task-009 with two nullable fields: int? openAssignmentCount and DateTime? assignmentDeadline. The badge should only render if: (1) OrgLabelsProvider indicates Blindeforbundet AND (2) the count is exactly 3 or 15.

Implement a private _AssignmentMilestoneBadge(int count, DateTime deadline) widget. Use a switch/if on count to select the badge color from design tokens — introduce two new tokens if they don't exist: colorMilestone3 (amber-ish) and colorMilestone15 (a distinct warm color, e.g., deep orange). Do NOT use hardcoded hex values. For OrgLabelsProvider integration, use context.watch() or equivalent — the org flag check must be reactive so the badge appears/disappears if the active org changes without widget reconstruction.

Format the deadline date using intl package's DateFormat('dd.MM.yyyy') — verify intl is already a project dependency before adding it. The Blindeforbundet office-fee trigger at 3rd and 15th assignments is a business rule described in the source documentation — add a code comment referencing this context for future maintainers.

Write widget tests using flutter_test with a mock OrgLabelsProvider injected via the widget tree (use ProviderScope overrides if using Riverpod, or InheritedWidget mock). Test 1: inject non-Blindeforbundet org — assert no badge widget in tree. Test 2: inject Blindeforbundet org, contact with openAssignmentCount=3 — assert badge is present and its decoration color matches the milestone-3 token. Test 3: inject Blindeforbundet org, contact with openAssignmentCount=15 — assert badge color matches milestone-15 token and color differs from milestone-3.

Test 4: inject Blindeforbundet org, contact with openAssignmentCount=7 — assert badge is absent (or present in neutral, per spec). Test 5: assert badge Semantics label contains the count and deadline string.