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/web/qr.go

@@ -1,9 +1,13 @@
 package web
 
 import (
+	"bytes"
 	"encoding/base64"
+	"image/png"
 	"net/http"
 
+	"github.com/boombuler/barcode"
+	"github.com/boombuler/barcode/code128"
 	qrcode "github.com/skip2/go-qrcode"
 )
 
@@ -17,6 +21,24 @@ func qrDataURI(content string, size int) (string, error) {
 	return "data:image/png;base64," + base64.StdEncoding.EncodeToString(png), nil
 }
 
+// barcodeDataURI renders content as a Code128 linear barcode PNG, scaled to
+// widthPx x heightPx, and returns it as a data: URI.
+func barcodeDataURI(content string, widthPx, heightPx int) (string, error) {
+	bc, err := code128.Encode(content)
+	if err != nil {
+		return "", err
+	}
+	scaled, err := barcode.Scale(bc, widthPx, heightPx)
+	if err != nil {
+		return "", err
+	}
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, scaled); err != nil {
+		return "", err
+	}
+	return "data:image/png;base64," + base64.StdEncoding.EncodeToString(buf.Bytes()), nil
+}
+
 // requestOrigin reconstructs the scheme+host the request arrived on, for
 // building absolute URLs (e.g. QR codes need a full URL, not a path).
 func requestOrigin(r *http.Request) string {