feat: implement soft delete, task editing fixes, and UI improvements

Backend:
- Add soft delete for spaces and projects (is_active flag)
- Add status_id and assignee_id to TaskUpdate schema
- Fix task PATCH endpoint to update status and assignee
- Add validation for assignee_id and status_id in task updates
- Fix health service to count tasks with "Blocked" status as blockers
- Filter out deleted spaces/projects from health dashboard
- Add workload cache invalidation on assignee changes

Frontend:
- Add delete confirmation dialogs for spaces and projects
- Fix UserSelect to display selected user name (valueName prop)
- Fix task detail modal to refresh data after save
- Enforce 2-level subtask depth limit in UI
- Fix timezone bug in date formatting (use local timezone)
- Convert NotificationBell from Tailwind to inline styles
- Add i18n translations for health, workload, settings pages
- Add parent_task_id to Task interface across components

OpenSpec:
- Archive add-delete-capability change

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/TaskDetailModal.tsx

@@ -24,6 +24,7 @@ interface Task {
   due_date: string | null
   time_estimate: number | null
   subtask_count: number
+  parent_task_id: string | null
   custom_values?: CustomValueResponse[]
 }
 
@@ -159,16 +160,13 @@ export function TaskDetailModal({
         priority: editForm.priority,
       }
 
-      if (editForm.status_id) {
-        payload.status_id = editForm.status_id
-      }
-      if (editForm.assignee_id) {
-        payload.assignee_id = editForm.assignee_id
-      } else {
-        payload.assignee_id = null
-      }
+      // Always send status_id (null to clear, or the value)
+      payload.status_id = editForm.status_id || null
+      // Always send assignee_id (null to clear, or the value)
+      payload.assignee_id = editForm.assignee_id || null
       if (editForm.due_date) {
-        payload.due_date = editForm.due_date
+        // Convert date string to datetime format for Pydantic 2
+        payload.due_date = `${editForm.due_date}T00:00:00`
       } else {
         payload.due_date = null
       }
@@ -322,13 +320,14 @@ export function TaskDetailModal({
               <TaskAttachments taskId={task.id} />
             </div>
 
-            {/* Subtasks Section */}
+            {/* Subtasks Section - only allow adding subtasks if this is not already a subtask (depth limit = 2) */}
             <div style={styles.section}>
               <SubtaskList
                 taskId={task.id}
                 projectId={task.project_id}
                 onSubtaskClick={onSubtaskClick}
                 onSubtaskCreated={onUpdate}
+                canAddSubtask={!task.parent_task_id}
               />
             </div>
           </div>
@@ -398,7 +397,8 @@ export function TaskDetailModal({
               <label style={styles.sidebarLabel}>{t('fields.assignee')}</label>
               {isEditing ? (
                 <UserSelect
-                  value={editForm.assignee_id}
+                  value={editForm.assignee_id || null}
+                  valueName={task.assignee_name}
                   onChange={handleAssigneeChange}
                   placeholder={t('common:labels.selectAssignee')}
                 />