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/Storage/BoardLoader.swift

@@ -838,6 +838,7 @@ public enum BoardLoader: Sendable {
                     icon: document.icon,
                     iconColor: document.iconColor,
                     hero: document.hero,
+                    labels: document.labels,
                     order: order,
                     attachments: entry.attachments,
                     commentCount: entry.commentCount,
@@ -1150,6 +1151,7 @@ public enum BoardLoader: Sendable {
                 icon: document.icon,
                 iconColor: document.iconColor,
                 hero: document.hero,
+                labels: document.labels,
                 order: order,
                 attachments: attachments,
                 commentCount: commentCount,