File ▸ Share… stages a board as a zip and hands it to the system share sheet

Duplicate's own posture — a faithful copy, `.git` the sole exclusion, attachments/comments/trash
carried verbatim — staged to a temp directory (BoardShareStager, ditto-zipped via
DittoZipArchiver) and presented through NSSharingServicePicker (BoardSharePresentation), anchored
to the board window's toolbar or its center. Follows Duplicate/Save as Template's flush-then-
cancellable-copy sequence under the banner's in-progress row (ShareBoardCommand, AppCommands.swift),
menu-validated on focus alone rather than the read-only lock (a share is a read, Print's own
posture) with the one carve-out an open inline title editor still needs. WriteOperation gains
.shareBoard for the banner vocabulary; 11-command-nexus.md's File menu table gains the row.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

Kanban/LiveStore/BannerCenter.swift

@@ -917,6 +917,12 @@ public final class BannerCenter {
             // pressed and the board is still exactly as it was: nothing was saved *over*, and the
             // failure is about the copy in the templates folder, not about this board's own files.
             if let title { "Couldn't save '\(title)' as a template" } else { "Couldn't save the board as a template" }
+        case let .shareBoard(title):
+            // The command's own word (File ▸ Share…), on `.saveAsTemplate`'s reasoning: the board
+            // itself is untouched by a share that fails partway — nothing here was saved *over* —
+            // so the sentence is about the staged copy that never reached the picker, not about
+            // this board's own files.
+            if let title { "Couldn't share '\(title)'" } else { "Couldn't share the board" }
         case let .importAttachment(filename):
             "Couldn't import '\(filename)'"
         case .listAttachments: