printers: printer installer map + install-batch endpoint

Rebuilds the classic printer-installer feature: pick printers on the shopfloor
map, download a .bat that installs them.

Backend (asset_routes.py): GET /api/printers/install-batch?printerids=1,2,3
returns a .bat attachment. Groups printers the way the classic installprinter.asp
did - HP/Xerox via the universal PrinterInstaller.exe /PRINTER="a,b,c", printers
with a .exe installpath via that installer /SILENT, and anything else (no
installpath, or a .zip) listed for manual install instead of being run blindly.
Download URLs derive from the site_base_url setting + the IIS-served /installers
folder (no hardcoded host). Reuses the existing install-list query shape.

Frontend: PrinterInstallerMap.vue - full-screen Leaflet shopfloor map (reuses
mapConfig), a marker per network printer at its mapx/mapy, click to toggle-select,
sidebar with the selection + an Install button that downloads the batch. Toplevel
route /printer-installer, printersApi.installList(), and an Installer Map button
on the printers list.

Tests: install-batch grouping (universal/specific/manual) + requires-ids.

plugins/printers/frontend/views/PrintersList.vue

@@ -3,6 +3,7 @@
     <div class="page-header">
       <h2>Printers</h2>
       <div class="header-actions">
+        <router-link to="/printer-installer" class="btn btn-secondary" target="_blank">Installer Map</router-link>
         <router-link to="/print/printer-qr" class="btn btn-secondary" target="_blank">Batch Print QR</router-link>
         <router-link to="/print/asset-label-batch/printer" class="btn btn-secondary" target="_blank">Print Labels</router-link>
         <router-link to="/printers/new" class="btn btn-primary">Add Printer</router-link>