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/pages/users_form.html

@@ -23,7 +23,8 @@
             {{if not .EditUser}}
             <div class="mb-3">
               <label class="form-label">Password</label>
-              <input type="password" class="form-control" name="password" required autocomplete="new-password">
+              <input type="password" class="form-control" name="password" autocomplete="new-password">
+              <div class="form-text">Leave blank for a Contact - they can't sign in, so no password is needed.</div>
             </div>
             {{end}}
             <div class="mb-3">
@@ -31,6 +32,7 @@
               <select class="form-select" name="role">
                 <option value="user" {{if .EditUser}}{{if eq .EditUser.Role "user"}}selected{{end}}{{end}}>User</option>
                 <option value="admin" {{if .EditUser}}{{if eq .EditUser.Role "admin"}}selected{{end}}{{end}}>Admin</option>
+                <option value="contact" {{if .EditUser}}{{if eq .EditUser.Role "contact"}}selected{{end}}{{end}}>Contact (no login - assignable only)</option>
               </select>
             </div>
             <div class="form-check mb-3">
@@ -47,7 +49,7 @@
         </div>
       </div>
 
-      {{if .EditUser}}
+      {{if and .EditUser .EditUser.CanLogin}}
       <div class="card">
         <div class="card-header py-2 fw-semibold">Change Password</div>
         <div class="card-body">