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/TaskViewerView.vue

@@ -111,6 +111,26 @@ function cycleStatus() {
   );
 }
 
+const forwardStatus: Record<TaskStatus, TaskStatus | null> = {
+  todo: "in_progress",
+  in_progress: "done",
+  done: null,
+};
+
+const advanceLabel = computed(() => {
+  const s = store.currentTask?.status as TaskStatus | undefined;
+  if (!s) return null;
+  const next = forwardStatus[s];
+  if (!next) return null;
+  return next === "in_progress" ? "→ In Progress" : "→ Done";
+});
+
+function advanceStatus() {
+  if (!store.currentTask) return;
+  const next = forwardStatus[store.currentTask.status as TaskStatus];
+  if (next) store.patchStatus(store.currentTask.id, next);
+}
+
 function isOverdue(): boolean {
   if (!store.currentTask?.due_date || store.currentTask.status === "done")
     return false;
@@ -192,6 +212,13 @@ const subTaskProgress = computed(() => {
           >
             Edit
           </router-link>
+          <button
+            v-if="advanceLabel"
+            class="btn-advance"
+            @click="advanceStatus"
+          >
+            {{ advanceLabel }}
+          </button>
           <button
             class="btn-convert"
             @click="convertToNote"
@@ -361,6 +388,20 @@ const subTaskProgress = computed(() => {
   border-color: var(--color-primary);
   color: var(--color-primary);
 }
+.btn-advance {
+  padding: 0.45rem 1rem;
+  background: linear-gradient(135deg, #6366f1, #4f46e5);
+  color: #fff;
+  border: none;
+  border-radius: var(--radius-sm);
+  cursor: pointer;
+  font-size: 0.9rem;
+  font-weight: 600;
+  transition: opacity 0.15s;
+}
+.btn-advance:hover {
+  opacity: 0.88;
+}
 .btn-convert {
   margin-left: auto;
   padding: 0.3rem 0.75rem;