Refactor: Create shared AttachmentService and AttachmentModel

- Created common AttachmentService for all modules (app/Services/)
- Created common AttachmentModel (app/Models/)
- Created common migration (app/Database/Migrations/)
- Updated Tasks module to use shared AttachmentService
- Added Twig functions: format_filesize(), get_file_icon()
- Removed duplicate TaskAttachmentModel and task_attachments migration
- AttachmentService can be used by any module: task, deal, contact, etc.

app/Modules/Tasks/Views/tasks/show.twig

@@ -155,13 +155,13 @@
                     <ul class="list-group list-group-flush mb-3" id="attachments-list">
                         {% for attachment in task.attachments %}
                             <li class="list-group-item d-flex align-items-center gap-2" data-attachment-id="{{ attachment.id }}">
-                                <i class="fa-solid fa-file text-muted"></i>
+                                <i class="fa-solid {{ get_file_icon(attachment.file_type) }} text-muted"></i>
                                 <a href="{{ base_url('/tasks/' ~ task.id ~ '/attachments/' ~ attachment.id ~ '/download') }}" 
                                    target="_blank" class="flex-grow-1 text-decoration-none">
                                     {{ attachment.file_name }}
                                 </a>
                                 <small class="text-muted">
-                                    {{ attachment.file_size|filesize }}
+                                    {{ format_filesize(attachment.file_size) }}
                                 </small>
                                 <button type="button" class="btn btn-outline-danger btn-sm"
                                         onclick="deleteAttachment({{ task.id }}, {{ attachment.id }})"