docs: clarify unit suffix handling in DN items panel header

The 'Total {sumQty}{unitSuffix}' header line now explicitly documents
how mixed / missing units are handled: suffix is omitted when all
items have empty/null units (operator defaults to 'EA'); when units
are mixed, no suffix is shown; when all items share a non-EA unit
(e.g. BOX), the suffix is shown.

Self-review pass; no other changes.

Co-Authored-By: Claude Code <noreply@anthropic.com>

docs/superpowers/specs/2026-09-01-deliveries-dn-items-expand-design.md

@@ -117,9 +117,15 @@ A bordered panel immediately below the existing `<div
 className="flex shrink-0 flex-col items-end gap-1.5">` button column,
 inside the same outer `<div className="flex items-start gap-3 ...">`:
 
-- **Header line**: "{n} items · Total {sumQty} EA" — right-aligned,
-  text-xs, muted foreground. When `n === 0`, the header line is
-  omitted; the empty-state takes its place.
+- **Header line**: "{n} items · Total {sumQty}{unitSuffix}" —
+  right-aligned, text-xs, muted foreground. `sumQty` is the
+  sum of `item.quantity` across the panel's items.
+  `unitSuffix` is empty when the items' units are mixed or
+  empty/null; otherwise it's a single shared unit string (e.g.
+  `" EA"` or `" BOX"`). When all items have unit "EA" or empty,
+  the suffix is omitted entirely (the operator's mental default
+  is "EA"). When `n === 0`, the header line is omitted; the
+  empty-state takes its place.
 - **Per item line** (`<div className="flex items-baseline gap-2">`):
   - Description: truncate + `title` for full text
     (`truncate`, `text-xs`)