Build the template chooser and Save as Template

The chooser completes its three tiers: bundled by template order, then
keyed user templates, then keyless boards by display name — and a
malformed user template still lists, by folder name with the loader's
own sentence on the row, never failing its neighbours. The store is
re-scanned on every presentation and on app activation, the Reveal
round trip made honest without watching a folder 09 deliberately
leaves unwatched; Reveal lives in the chooser's header and mints the
store on first press. Save as Template repeats Duplicate's sequence —
progress row with Cancel, flush, detached cancellable copy — through
the engine: mint the store, read the next user order before the copy
can count itself, Finder-ladder the name, copy excluding .git and
.trash/, then stamp the whole template: mapping on the landed copy
through updateIndex, with no bracket because the copy lives outside
every watched board. Folder attributes deliberately don't carry — the
one lock the command stays live under is the read-only-DMG one, and
carrying its mode bits would mint a read-only template in the user's
own store; the command gates instead on the real hazard, unsaved card
content. A signpost names the template only when the ladder renamed
it. One name ladder now serves Duplicate and the store.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/LiveStore/BannerCenter.swift

@@ -671,6 +671,11 @@ public final class BannerCenter {
             if let title { "Couldn't rename '\(title)'" } else { "Couldn't rename the item" }
         case let .duplicateBoard(title):
             if let title { "Couldn't duplicate '\(title)'" } else { "Couldn't duplicate the board" }
+        case let .saveAsTemplate(title):
+            // The command's own words (File ▸ Save as Template), because that is the button they
+            // 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 .importAttachment(filename):
             "Couldn't import '\(filename)'"
         case .listAttachments: