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.

internal/assets/assets.go

@@ -246,6 +246,15 @@ func (s *Store) AssignUser(ctx context.Context, id int64, userID *int64) error {
 	return err
 }
 
+// SetStatus changes only an asset's status, independent of the full edit
+// form - used by the bulk "Edit Status" multi-select action.
+func (s *Store) SetStatus(ctx context.Context, id, statusID int64) error {
+	_, err := s.db.ExecContext(ctx,
+		`UPDATE assets SET status_id = ?, updated_at = strftime('%Y-%m-%dT%H:%M:%fZ','now') WHERE id = ?`,
+		statusID, id)
+	return err
+}
+
 var ErrQuantityNotTracked = errors.New("quantity is not tracked for unique item types")
 
 // AdjustQuantity increments or decrements an asset's quantity by delta,