🐛(components): add missing button types and keys

Add type="button" to the EditMobility and UploadStudents buttons
to prevent default form submission behavior.
Include a key prop on Student components in Promotion for stable list rendering.

routes/(apps)/mobility/(_islands)/EditMobility.tsx

@@ -240,7 +240,7 @@ export default function EditMobility() {
           </table>
         </div>
       ))}
-      <button onClick={handleSave} disabled={isSaving}>
+      <button type="button" onClick={handleSave} disabled={isSaving}>
         {isSaving ? "Saving..." : "Confirm"}
       </button>
     </section>

routes/(apps)/students/(_components)/Promotion.tsx

@@ -22,7 +22,7 @@ export default function Promotion(props: PromotionProps) {
         <tbody>
           {props.students
             .filter((student) => student.promotionId === props.promo.id)
-            .map((student) => <Student student={student} />)}
+            .map((student) => <Student key={student.id} student={student} />)}
         </tbody>
       </table>
     </div>

routes/(apps)/students/(_islands)/UploadStudents.tsx

@@ -104,7 +104,7 @@ export default function UploadStudents() {
   return (
     <>
       <input type="file" accept=".xlsx, .xls" onChange={handleFileChange} />
-      <button onClick={confirmUpload}>Confirm Upload</button>
+      <button type="button" onClick={confirmUpload}>Confirm Upload</button>
       <p>{statusMessage.value}</p>
     </>
   );