Add section field to boss battles for run progression dividers

Adds a nullable `section` column to boss battles (e.g. "Main Story",
"Endgame") with dividers rendered in the run view between sections.
Admin UI gets a Section column in the table and a text input in the form.

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

frontend/src/components/admin/BossBattleFormModal.tsx

@@ -43,6 +43,7 @@ export function BossBattleFormModal({
   const [order, setOrder] = useState(String(boss?.order ?? nextOrder))
   const [afterRouteId, setAfterRouteId] = useState(String(boss?.afterRouteId ?? ''))
   const [location, setLocation] = useState(boss?.location ?? '')
+  const [section, setSection] = useState(boss?.section ?? '')
   const [spriteUrl, setSpriteUrl] = useState(boss?.spriteUrl ?? '')
 
   const handleSubmit = (e: FormEvent) => {
@@ -56,6 +57,7 @@ export function BossBattleFormModal({
       order: Number(order),
       afterRouteId: afterRouteId ? Number(afterRouteId) : null,
       location,
+      section: section || null,
       spriteUrl: spriteUrl || null,
     })
   }
@@ -146,6 +148,17 @@ export function BossBattleFormModal({
         </div>
       </div>
 
+      <div>
+        <label className="block text-sm font-medium mb-1">Section</label>
+        <input
+          type="text"
+          value={section}
+          onChange={(e) => setSection(e.target.value)}
+          placeholder="e.g. Main Story, Endgame"
+          className="w-full px-3 py-2 border rounded-md dark:bg-gray-700 dark:border-gray-600"
+        />
+      </div>
+
       <div>
         <label className="block text-sm font-medium mb-1">Position After Route</label>
         <select