Bulk multi-select actions, barcode format, batch printing, site name on login

Multi-select (Ctrl/Cmd+click toggle, Shift+click range, right-click the
active selection for bulk actions - same conventions as a native file
manager):
- Print Labels (N) / Export CSV (N) for any mix of individually-picked
  assets and whole folders. Labels/export now take asset_ids/folder_ids
  instead of a single asset_id/folder_id.
- Edit Status..., Check Out..., Duplicate, Move to..., and Delete, bulk
  versions of the existing single-asset actions. Checkout/Move modals
  were generalized to accept an array of ids instead of just one; status
  editing got its own lightweight POST /app/assets/{id}/status endpoint
  (assets.SetStatus) so a bulk status change doesn't need to resend every
  other field the way a full PATCH would.

Labels:
- Choose QR (a /scan/<tag> URL, for a phone camera) or Code128 barcode
  (the bare Asset ID, for a USB scanner typing into the search box) via a
  format selector on the print page. New barcodeDataURI using
  github.com/boombuler/barcode alongside the existing QR generator.
- "Start at #" lets printing resume partway into an already-used sheet
  instead of wasting it on a fresh one.
- Added the Avery 5260 label template (same grid as 5160, rounded
  corners, so it isn't flagged approximate).

Also: the configured Site Name now shows on the login page too, not just
the authenticated navbar.

web/static/css/app.css

@@ -77,6 +77,15 @@ ul.tree-children {
   font-weight: 500;
 }
 
+/* Ctrl/Shift-click multi-selection (right-click one of these for bulk
+   actions). Distinct from .selected, which marks the single item shown in
+   the main panel - a row can carry both at once. */
+.tree-row.multi-selected {
+  background-color: #fff3cd;
+  outline: 1px solid #ffe69c;
+  outline-offset: -1px;
+}
+
 .tree-row.drop-target {
   outline: 2px dashed #0d6efd;
   outline-offset: -2px;