Add milestone data layer and redesign Projects tab

Milestone data layer:
- lib/data/models/milestone.dart: Milestone model with progress fields
  (total, completed, pct) from backend progress endpoint
- lib/data/api/milestones_api.dart: CRUD client for
  /api/projects/:id/milestones
- lib/data/repositories/milestones_repository.dart: thin repository wrapper
- lib/providers/milestones_provider.dart: FutureProvider.family keyed by
  project ID — lazily fetched per project when expanded
- api_client_provider.dart: registers milestonesApiProvider and
  milestonesRepositoryProvider

Projects tab redesign (project_list_screen.dart):
- Each project is now an ExpansionTile showing open task count in subtitle
- Expanding a project fetches its milestones and groups tasks under milestone
  headers with inline progress bar (completed/total count)
- Tasks show status icon, due date with overdue highlighting, priority dot
- 'No milestone' section at bottom for unassigned tasks
- Tapping a task navigates to the task editor
- Long-press on project title opens archive/complete/delete bottom sheet
- 'New task' button at bottom of each expanded project section

Bump version to 26.03.02+1

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/data/models/milestone.dart

@@ -0,0 +1,41 @@
+class Milestone {
+  final int id;
+  final int projectId;
+  final String title;
+  final String? description;
+  final String status; // active | completed | archived
+  final int orderIndex;
+  final int total;
+  final int completed;
+  final double pct;
+  final DateTime createdAt;
+  final DateTime updatedAt;
+
+  const Milestone({
+    required this.id,
+    required this.projectId,
+    required this.title,
+    this.description,
+    required this.status,
+    required this.orderIndex,
+    required this.total,
+    required this.completed,
+    required this.pct,
+    required this.createdAt,
+    required this.updatedAt,
+  });
+
+  factory Milestone.fromJson(Map<String, dynamic> json) => Milestone(
+        id: json['id'] as int,
+        projectId: json['project_id'] as int,
+        title: json['title'] as String? ?? '',
+        description: json['description'] as String?,
+        status: json['status'] as String? ?? 'active',
+        orderIndex: json['order_index'] as int? ?? 0,
+        total: json['total'] as int? ?? 0,
+        completed: json['completed'] as int? ?? 0,
+        pct: (json['pct'] as num?)?.toDouble() ?? 0.0,
+        createdAt: DateTime.parse(json['created_at'] as String),
+        updatedAt: DateTime.parse(json['updated_at'] as String),
+      );
+}