feat: replace status dropdown with one-click toggle buttons

Replace <select> in side panel with 4 toggle buttons (To Do / Doing /
Pending / Done). Clicking a button immediately sends the PATCH status
API call. Active button is highlighted with status-specific colors and
shadow. saveTask now only handles title/description changes.

static/css/tasks.css

@@ -97,6 +97,72 @@
     border-color: #28a745;
 }
 
+.status-toggle {
+    display: flex;
+    gap: 0.5rem;
+    flex-wrap: wrap;
+}
+
+.status-btn {
+    padding: 0.5rem 1rem;
+    border: 2px solid #ddd;
+    border-radius: 6px;
+    cursor: pointer;
+    font-size: 0.875rem;
+    font-weight: 500;
+    transition: all 0.15s ease;
+    background: #f8f9fa;
+    color: #666;
+}
+
+.status-btn.todo:hover {
+    border-color: #667eea;
+    color: #667eea;
+}
+
+.status-btn.todo.active {
+    background: #f8f9fa;
+    border-color: #667eea;
+    color: #667eea;
+    box-shadow: 0 0 0 2px rgba(102, 126, 234, 0.2);
+}
+
+.status-btn.doing:hover {
+    border-color: #ffc107;
+    color: #b8860b;
+}
+
+.status-btn.doing.active {
+    background: #fff3cd;
+    border-color: #ffc107;
+    color: #856404;
+    box-shadow: 0 0 0 2px rgba(255, 193, 7, 0.2);
+}
+
+.status-btn.pending:hover {
+    border-color: #2196F3;
+    color: #2196F3;
+}
+
+.status-btn.pending.active {
+    background: #e7f3ff;
+    border-color: #2196F3;
+    color: #0d6efd;
+    box-shadow: 0 0 0 2px rgba(33, 150, 243, 0.2);
+}
+
+.status-btn.done:hover {
+    border-color: #28a745;
+    color: #28a745;
+}
+
+.status-btn.done.active {
+    background: #d4edda;
+    border-color: #28a745;
+    color: #155724;
+    box-shadow: 0 0 0 2px rgba(40, 167, 69, 0.2);
+}
+
 .task-item:hover {
     background: #e9ecef;
 }