fix: location dropdowns rendered blank (wrong field) + require printer model

The location option label read l.location, but the Location.to_dict() field is
locationname, so every option rendered blank - the dropdown looked empty and
"massive" (a long list of blank rows). Fixed across all five affected forms:
printers, computers, network devices, network device form, and the subnets
location filter. Other .location uses (printer-driver URL, search-result label,
report bylocation key) are legitimately different fields, left alone.

Also require a model on the printer form: asterisk + required attr, plus a JS
guard in savePrinter (the native required is skipped while the select is
disabled with no vendor picked) that points the user at the vendor first.

plugins/printers/frontend/views/PrinterForm.vue

@@ -144,12 +144,13 @@
           </div>
 
           <div class="form-group">
-            <label for="modelnumberid">Model</label>
+            <label for="modelnumberid">Model *</label>
             <select
               id="modelnumberid"
               v-model="form.modelnumberid"
               class="form-control"
               :disabled="!form.vendorid"
+              required
             >
               <option value="">{{ form.vendorid ? 'Select model...' : 'Select a vendor first' }}</option>
               <option
@@ -179,7 +180,7 @@
               :key="l.locationid"
               :value="l.locationid"
             >
-              {{ l.location }}
+              {{ l.locationname }}
             </option>
           </select>
         </div>
@@ -549,6 +550,16 @@ function clearMapPosition() {
 
 async function savePrinter() {
   error.value = ''
+
+  // Model is required. The native `required` on the select is skipped while it
+  // is disabled (no vendor picked), so guard here too and point at the vendor.
+  if (!form.value.modelnumberid) {
+    error.value = form.value.vendorid
+      ? 'Model is required.'
+      : 'Select a vendor, then a model (model is required).'
+    return
+  }
+
   saving.value = true
 
   try {