Add asset checkout, quantity/uniqueness, contacts, and QR labels

Checkout:
- assets.AssignUser + POST /app/assets/{id}/checkout assign/unassign an
  asset's user independent of the full edit form. New shared
  #checkoutModal, wired from a "Check Out..." context-menu action, backed
  by GET /app/assets/{id} and GET /app/users/options JSON endpoints.

Quantity / uniqueness:
- Item types gain an IsUnique flag (checked by default, matching prior
  behavior). Assets gain a Quantity column, fixed at 1 for unique types
  and adjustable via +/- steppers (POST /app/assets/{id}/quantity) for
  non-unique ones. Flipping a type back to unique clamps existing assets'
  quantity back to 1. CSV import/export gained a Quantity column.
  Migration 0003_quantity.sql.

Contacts:
- New models.RoleContact: an assignable person who cannot sign in
  (User.CanLogin). Migration 0004_contacts.sql rebuilds the users table
  (SQLite can't alter a CHECK constraint in place) to allow the role.
  Users form/list updated; contact creation auto-generates an unusable
  password since the login path is blocked by role regardless.

QR asset tags and printable labels:
- Each asset's QR code (via github.com/skip2/go-qrcode, pure Go) encodes
  an absolute /scan/<tag> URL; GET /scan/{tag} resolves it and redirects
  into /inventory?asset=<id>, which app.js picks up on load to
  auto-select that asset - works from a phone camera or a USB
  scanner-as-keyboard-wedge pointed at the address bar.
- GET /labels renders a print-ready sheet (right-click an asset for one
  label, a folder for every asset in it) with a template switcher between
  Avery 5160 (verified dimensions) and 61526 asset tag labels (best-effort
  layout, flagged as such on the page).

Also refreshed README to reflect the right-click-first tree UX and all of
the above.

web/templates/partials/asset_fields.html

@@ -1,7 +1,20 @@
 {{define "asset_fields_only"}}
-{{if .}}
+{{if not .IsUnique}}
+<div class="mb-3">
+  <label class="form-label" for="assetFormQuantity">Quantity</label>
+  <div class="input-group" style="max-width: 220px">
+    <button class="btn btn-outline-secondary" type="button" data-action="quantity-step" data-step="-1">&minus;</button>
+    <input type="number" class="form-control text-center" name="quantity" id="assetFormQuantity" min="0" step="1" value="{{.Quantity}}">
+    <button class="btn btn-outline-secondary" type="button" data-action="quantity-step" data-step="1">+</button>
+  </div>
+  <div class="form-text">This item type isn't Unique, so it's tracked by quantity instead of one row per unit.</div>
+</div>
+{{else}}
+<input type="hidden" name="quantity" value="1">
+{{end}}
+{{if .Fields}}
 <h6 class="mt-3 mb-2">Type-Specific Fields</h6>
-{{range .}}
+{{range .Fields}}
 {{$fv := .}}
 <div class="mb-2">
   <label class="form-label" for="field-{{.Def.ID}}">{{.Def.Name}}{{if .Def.Required}} <span class="text-danger">*</span>{{end}}</label>

web/templates/partials/asset_form.html

@@ -49,7 +49,7 @@
       <select class="form-select" name="assigned_user_id">
         <option value="">Unassigned</option>
         {{range .Users}}
-        <option value="{{.ID}}" {{if eq $selectedUser .ID}}selected{{end}}>{{.Username}}</option>
+        <option value="{{.ID}}" {{if eq $selectedUser .ID}}selected{{end}}>{{.Username}}{{if eq .Role "contact"}} (Contact){{end}}</option>
         {{end}}
       </select>
     </div>
@@ -60,7 +60,7 @@
     </div>
 
     <div class="col-12" id="asset-fields-container">
-      {{template "asset_fields_only" .Fields}}
+      {{template "asset_fields_only" (dict "IsUnique" .SelectedITUnique "Quantity" .Quantity "Fields" .Fields)}}
     </div>
   </div>
 

web/templates/partials/modals.html

@@ -128,6 +128,29 @@
   </div>
 </div>
 
+<!-- Check out / check in asset -->
+<div class="modal fade" id="checkoutModal" tabindex="-1">
+  <div class="modal-dialog">
+    <div class="modal-content">
+      <div class="modal-header">
+        <h5 class="modal-title" id="checkoutModalTitle">Check Out Asset</h5>
+        <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
+      </div>
+      <div class="modal-body">
+        <p class="text-body-secondary small mb-3" id="checkoutCurrent"></p>
+        <label class="form-label">Assigned User</label>
+        <select class="form-select" id="checkoutUserSelect">
+          <option value="">Unassigned (check in)</option>
+        </select>
+      </div>
+      <div class="modal-footer">
+        <button type="button" class="btn btn-outline-secondary" data-bs-dismiss="modal">Cancel</button>
+        <button type="button" class="btn btn-primary" id="confirmCheckoutBtn">Save</button>
+      </div>
+    </div>
+  </div>
+</div>
+
 <!-- Import CSV into a specific folder (right-click a folder > Import CSV...) -->
 <div class="modal fade" id="importModal" tabindex="-1">
   <div class="modal-dialog">

web/templates/partials/panel_asset.html

@@ -25,7 +25,19 @@
         <div class="card-header py-2 fw-semibold">Details</div>
         <table class="table table-sm mb-0">
           <tbody>
-            <tr><th class="w-25">Assigned User</th><td>{{if .Asset.AssignedUsername}}{{.Asset.AssignedUsername}}{{else}}<span class="text-body-secondary">Unassigned</span>{{end}}</td></tr>
+            <tr><th class="w-25">Assigned User</th><td>{{if .Asset.AssignedUsername}}{{.Asset.AssignedUsername}}{{else}}<span class="text-body-secondary">Unassigned &mdash; right-click above to check out</span>{{end}}</td></tr>
+            {{if not .Asset.ItemTypeIsUnique}}
+            <tr>
+              <th>Quantity</th>
+              <td>
+                <div class="input-group input-group-sm" style="max-width: 150px" data-quantity-stepper data-asset-id="{{.Asset.ID}}">
+                  <button class="btn btn-outline-secondary" type="button" data-action="panel-quantity-step" data-step="-1">&minus;</button>
+                  <span class="form-control text-center" data-quantity-value>{{.Asset.Quantity}}</span>
+                  <button class="btn btn-outline-secondary" type="button" data-action="panel-quantity-step" data-step="1">+</button>
+                </div>
+              </td>
+            </tr>
+            {{end}}
             <tr><th>Created</th><td>{{fmtDate .Asset.CreatedAt}}</td></tr>
             <tr><th>Updated</th><td>{{fmtDate .Asset.UpdatedAt}}</td></tr>
           </tbody>

web/templates/partials/panel_folder.html

@@ -38,13 +38,23 @@
   {{if .Assets}}
   <div class="table-responsive">
     <table class="table table-sm table-hover asset-table">
-      <thead><tr><th>Asset ID</th><th>Name</th><th>Type</th><th>Status</th><th>Assigned</th></tr></thead>
+      <thead><tr><th>Asset ID</th><th>Name</th><th>Type</th><th>Qty</th><th>Status</th><th>Assigned</th></tr></thead>
       <tbody>
         {{range .Assets}}
         <tr class="asset-row" data-id="{{.ID}}">
           <td>{{.AssetTag}}</td>
           <td><i class="bi {{iconOr .ItemTypeIcon "bi-box-seam"}} me-1"></i>{{.Name}}</td>
           <td>{{.ItemTypeName}}</td>
+          <td>
+            {{if .ItemTypeIsUnique}}<span class="text-body-secondary">&mdash;</span>
+            {{else}}
+            <div class="input-group input-group-sm" style="width:6.5rem" data-quantity-stepper data-asset-id="{{.ID}}">
+              <button class="btn btn-outline-secondary py-0" type="button" data-action="table-quantity-step" data-step="-1">&minus;</button>
+              <span class="form-control text-center py-0" data-quantity-value>{{.Quantity}}</span>
+              <button class="btn btn-outline-secondary py-0" type="button" data-action="table-quantity-step" data-step="1">+</button>
+            </div>
+            {{end}}
+          </td>
           <td><span class="badge text-bg-{{.StatusColor}}">{{.StatusName}}</span></td>
           <td>{{if .AssignedUsername}}{{.AssignedUsername}}{{else}}<span class="text-body-secondary">&mdash;</span>{{end}}</td>
         </tr>