fix(flow): replace duplicate period dropdown with preset buttons

Period selector had both preset buttons (3/7/30 zile) and a dropdown
with overlapping options. Per plan Commit 6, preset buttons are the
single control: Azi / 3 zile / 7 zile / 30 zile / Custom. Visible
on all screen sizes with horizontal scroll on mobile.

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

api/app/static/css/style.css

@@ -1039,6 +1039,8 @@ tr.mapping-deleted td {
     display: flex;
     gap: 2px;
     align-items: center;
+    overflow-x: auto;
+    flex-shrink: 0;
 }
 .preset-btn {
     background: var(--surface);