Mark unfrozen Supply archive posts and open them in the Supply run

The Supply archive presented every post as a frozen, read-only list, even a
past week's Supply run that week_rollover had not frozen and that the
Supply run route could still change. Each archive post now carries frozen;
a post not yet frozen is marked (Current, or Still open for a past week)
and links to the Supply run, where it can still change. Only frozen posts
open read-only in the archive.

Maestro task 29b629aa-1ad9-48a3-be75-0380b645d4fe

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/shopping.go

@@ -222,8 +222,9 @@ type archivePeriod struct {
 }
 
 // archivePost is one week's Supply run in the Supply archive. label is
-// the week label, which stands in for title while it is null, and
-// onHandCount counts the items marked "have it".
+// the week label, which stands in for title while it is null,
+// onHandCount counts the items marked "have it", and frozen is false
+// while the list can still be changed from the Supply run.
 type archivePost struct {
 	WeekStart   string   `json:"weekStart"`
 	Label       string   `json:"label"`
@@ -233,6 +234,7 @@ type archivePost struct {
 	OnHandCount int      `json:"onHandCount"`
 	Dinners     []string `json:"dinners"`
 	Current     bool     `json:"current"`
+	Frozen      bool     `json:"frozen"`
 }
 
 // getSupplyArchive answers GET /shopping/archive?by=week|month|year with
@@ -262,6 +264,7 @@ func getSupplyArchive(logger *slog.Logger, svc *shopping.Service) http.HandlerFu
 					OnHandCount: post.OnHand,
 					Dinners:     post.Dinners,
 					Current:     post.Current,
+					Frozen:      post.Frozen,
 				}
 			}
 			out.Periods[i] = archivePeriod{Key: p.Key, Heading: p.Heading, Posts: posts}