Style schedule-start checkboxes to match Angular

The two filter toggles ('Только прямые рейсы' / 'Показать обратные рейсы')
were rendering as the browser default 16px native checkboxes, which look
inconsistent across browsers and thin against the rest of the page.
Add an appearance:none + custom box with a blue tick mark when checked,
matching Angular's filter card.

src/features/schedule/components/ScheduleStartPage.scss

@@ -131,6 +131,57 @@
       margin-bottom: vars.$label-margin-bottom;
     }
 
+    // Give the checkbox a proper box + sliding-knob style, matching
+    // Angular's schedule filter. Native `input[type="checkbox"]` is too
+    // thin at 16px and its accented colour differs between browsers.
+    label:has(input[type="checkbox"]) {
+      display: flex;
+      flex-direction: row;
+      align-items: center;
+      gap: 10px;
+      font-weight: normal;
+      margin-bottom: 0;
+      cursor: pointer;
+      font-size: 14px;
+      color: #222;
+    }
+
+    input[type="checkbox"] {
+      appearance: none;
+      -webkit-appearance: none;
+      width: 18px;
+      height: 18px;
+      border: 1.5px solid #d0d5dd;
+      border-radius: 3px;
+      background: #fff;
+      position: relative;
+      cursor: pointer;
+      flex-shrink: 0;
+      transition: background-color 120ms ease, border-color 120ms ease;
+
+      &:checked {
+        background-color: colors.$blue;
+        border-color: colors.$blue;
+
+        &::after {
+          content: "";
+          position: absolute;
+          top: 1px;
+          left: 5px;
+          width: 5px;
+          height: 10px;
+          border: solid #fff;
+          border-width: 0 2px 2px 0;
+          transform: rotate(45deg);
+        }
+      }
+
+      &:focus-visible {
+        outline: 2px solid colors.$blue;
+        outline-offset: 2px;
+      }
+    }
+
     input[type="text"],
     input[type="date"] {
       @include shadows.control-border-shadow();