A reserved key comes to life — the card window's sidebar grows a Labels section, and labels stops being somebody else's

`labels` has been a reserved tracker key since the rewrite: preserved verbatim, never
interpreted, drawn only as an anonymous row in the Details section beside `assignees` and
`due`. The owner's cards claim it for first-party use, so it joins the schema — read
leniently (a list of names, a bare scalar coercing to one, a mapping malformed and
preserved), written canonically (a quoted flow list in the order the user arranged, no
auto-sort), and removed outright when the last label goes, the way an expanded lane drops
`collapsed`.

Identity is case-insensitive and display is case-preserving, so a card carries `bug` once
however many ways the board spells it, and entries the reading cannot name ride through the
write untouched at the tail.

The section sits second, above Details — which is the point rather than a layout preference:
Details is where keys the app does *not* own are shown, and this key just stopped being one.
Rows rather than chips, because the sidebar is twenty-six characters wide. The add field
autocompletes against the board's own used-labels universe, derived from every live and
trashed card with no store beside the files, and says out loud when Return would mint a word
the board has never used.

Writes ride a `.relabel` operation of their own, because the commit composer has said
"Relabel card 'X'" since long before there was a control to press — and now the undo row says
it too.

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

Kanban/LiveStore/BannerCenter.swift

@@ -927,6 +927,15 @@ public final class BannerCenter {
             if let title { "Couldn't update '\(title)' to the current format" } else { "Couldn't update an item to the current format" }
         case let .style(title):
             if let title { "Couldn't restyle '\(title)'" } else { "Couldn't restyle the item" }
+        case let .relabel(title):
+            // **"relabel", in both directions** — `.collapse`/`.expand`'s two-word split turned down
+            // for the reason `WriteOperation.relabel` gives: adding and removing a label are the same
+            // control pressed twice, so one verb is the honest one. The word is 06-history-undo.md's
+            // own for a change to this key ("Relabel card 'X'"), which the commit composer has spoken
+            // since before there was a control to press. It says "the card" rather than "the item"
+            // where every other row here says the latter, because labels are a card field and nothing
+            // else can reach this sentence.
+            if let title { "Couldn't relabel '\(title)'" } else { "Couldn't relabel the card" }
         case .setBoardBackground:
             // **"generate", because that is the button they pressed**, and no title because there is
             // one board and they are looking at it. It deliberately says nothing about the *file* —