Task UX: forward-only advance button, auto edit mode, reload on save

TaskViewerView:
- Add '→ In Progress' / '→ Done' advance button in toolbar (forward-only,
  hidden when status is already done)
- Styled with primary indigo gradient to distinguish from secondary actions

TaskEditorView:
- showPreview now defaults false; set to true after load only when body
  has content — empty tasks open directly in Write mode
- Save on an existing task now calls window.location.reload() so the
  preview/edit mode re-evaluates from the freshly persisted body

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

frontend/src/views/TaskEditorView.vue

@@ -98,7 +98,7 @@ async function toggleSubTask(sub: SubTask) {
     toast.show("Failed to update sub-task", "error");
   }
 }
-const showPreview = ref(true);
+const showPreview = ref(false);
 const sidebarOpen = ref(true);
 const editorRef = ref<InstanceType<typeof TiptapEditor> | null>(null);
 const titleRef = ref<HTMLInputElement | null>(null);
@@ -266,6 +266,8 @@ onMounted(async () => {
       savedProjectId = projectId.value;
       savedMilestoneId = milestoneId.value;
       savedParentId = parentId.value;
+      // Start in preview mode only if the task already has body content
+      showPreview.value = body.value.trim().length > 0;
     }
     loadSubTasks();
   } else {
@@ -304,6 +306,7 @@ async function save() {
       savedParentId = parentId.value;
       dirty.value = false;
       toast.show("Task saved");
+      window.location.reload();
     } else {
       const task = await store.createTask(data);
       dirty.value = false;