feat(import): input template download with three sheets (spec 2.3 step 4)

- buildTemplateWorkbook: 교각 · IP · 받침 sheets with the headers the parser
  reads; value cells left empty so no blank-name rows raise E05
- dialog button 입력 템플릿 다운로드 (.xlsx)
- tests: sheet order, parser recognition, write-and-read round trip

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

index.html

@@ -117,6 +117,7 @@
               <input id="file" type="file" accept=".xlsx" hidden />
             </div>
             <p id="file-name" class="muted small"></p>
+            <button id="download-template" class="btn small" type="button">입력 템플릿 다운로드 (.xlsx)</button>
           </section>
 
           <section>