feat: add pinned tag functionality and UI enhancements

- Introduced `isPinned` property to the `Tag` model for marking main objectives.
- Updated `TagForm` to include a checkbox for setting tags as pinned, enhancing user interaction.
- Enhanced `KanbanBoardContainer` to display pinned tasks in a dedicated `ObjectivesBoard`, improving task visibility.
- Modified `KanbanFilters` to support filtering by pinned tags, streamlining task management.
- Adjusted `TasksContext` to separate pinned tasks from regular tasks for better organization.

prisma/schema.prisma

@@ -36,10 +36,11 @@ model Task {
 }
 
 model Tag {
-  id       String    @id @default(cuid())
-  name     String    @unique
-  color    String    @default("#6b7280")
-  taskTags TaskTag[]
+  id         String    @id @default(cuid())
+  name       String    @unique
+  color      String    @default("#6b7280")
+  isPinned   Boolean   @default(false) // Tag pour objectifs principaux
+  taskTags   TaskTag[]
   
   @@map("tags")
 }