[CLAUDE] App+Infra+Api+FE-Admin: PDF export (LibreOffice headless)

Pipeline: template.docx → FormRenderer fill placeholders → LibreOffice
soffice --headless --convert-to pdf → PDF byte[] → File() stream to
browser.

Clean-arch split:
- Application: IPdfConverter abstraction (swap to QuestPDF/Aspose later
  without touching caller).
- Infrastructure: LibreOfficePdfConverter — shells out to soffice.exe
  path from config (Pdf:SofficePath, default
  `C:\Program Files\LibreOffice\program\soffice.exe` on Windows).
  Per-request temp workDir để tránh filename collision + -env:
  UserInstallation isolate mỗi conversion (chống "soffice already
  running" khi concurrent). Timeout 60s (configurable). Best-effort
  cleanup. Kill entire process tree nếu timeout.
- Application: ExportTemplatePdfCommand — reuses existing FormRenderer
  + pipes bytes through IPdfConverter. Same data dict signature as
  Render để UI code share.
- Api: POST /api/forms/templates/{id}/export-pdf (same JSON body as
  /render, returns PDF stream).

FE:
- useExport hook chung cho 2 endpoints (DRY render + export-pdf mutations)
- Render dialog thêm nút "Tải PDF" (outline variant) cạnh "Tải file gốc".
  Disabled khi mutation khác đang chạy.
- Hướng dẫn dialog nâng cấp: "file gốc để edit Word/Excel, PDF để
  in/gửi không chỉnh sửa được".

Ops: scripts/install-libreoffice.ps1 — silent MSI install 25.8.6 cho
VPS (đã chạy trên prod).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Backend/SolutionErp.Api/Controllers/FormsController.cs

@@ -29,6 +29,13 @@ public class FormsController(IMediator mediator) : ControllerBase
         return File(result.Content, result.ContentType, result.FileName);
     }
 
+    [HttpPost("templates/{id:guid}/export-pdf")]
+    public async Task<IActionResult> ExportPdf(Guid id, [FromBody] Dictionary<string, string?> data, CancellationToken ct)
+    {
+        var result = await mediator.Send(new ExportTemplatePdfCommand(id, data), ct);
+        return File(result.Content, result.ContentType, result.FileName);
+    }
+
     // ==================== Admin CRUD ====================
 
     // Upload new template — multipart form with file + metadata fields