feat: schedule modal validation + time range guard

- Validate trainer, type, and time before saving
- Show overlap warnings for conflicting classes
- Reset end time when start time exceeds it
- Block setting end time earlier than start
- Remove day change hints from modal

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

src/app/admin/_components/FormField.tsx

@@ -140,6 +140,21 @@ export function TimeRangeField({ label, value, onChange, onBlur }: TimeRangeFiel
     }
   }
 
+  function handleStartChange(newStart: string) {
+    // Reset end if start >= end
+    if (newStart && end && newStart >= end) {
+      update(newStart, "");
+    } else {
+      update(newStart, end);
+    }
+  }
+
+  function handleEndChange(newEnd: string) {
+    // Ignore if end <= start
+    if (start && newEnd && newEnd <= start) return;
+    update(start, newEnd);
+  }
+
   return (
     <div>
       <label className="block text-sm text-neutral-400 mb-1.5">{label}</label>
@@ -147,7 +162,7 @@ export function TimeRangeField({ label, value, onChange, onBlur }: TimeRangeFiel
         <input
           type="time"
           value={start}
-          onChange={(e) => update(e.target.value, end)}
+          onChange={(e) => handleStartChange(e.target.value)}
           onBlur={onBlur}
           className="flex-1 rounded-lg border border-white/10 bg-neutral-800 px-3 py-2.5 text-white outline-none focus:border-gold transition-colors"
         />
@@ -155,7 +170,7 @@ export function TimeRangeField({ label, value, onChange, onBlur }: TimeRangeFiel
         <input
           type="time"
           value={end}
-          onChange={(e) => update(start, e.target.value)}
+          onChange={(e) => handleEndChange(e.target.value)}
           onBlur={onBlur}
           className="flex-1 rounded-lg border border-white/10 bg-neutral-800 px-3 py-2.5 text-white outline-none focus:border-gold transition-colors"
         />