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/App/CardWindowHost.swift

@@ -492,6 +492,9 @@ struct CardWindowHost: View {
                 // editor feeds are app-wide state (02-architecture.md § Per-board app state), so
                 // they come from the model every window shares rather than from this board's store.
                 recents: appModel.styleRecents,
+                // The same argument one field over: the labels this user reached for lately are
+                // app-wide state (`LabelRecents`), so they come from the model every window shares.
+                labelRecents: appModel.labelRecents,
                 cardFolder: Self.cardFolder(root: store.rootURL, placement: placement),
                 bodyPresentation: bodyPresentation,
                 bodySession: session.body,