feat(labels): the Label Designer — QSO and address labels for paper QSL

The designer's data model lives in internal/labels: STOCKS are the physical
roll in the printer (geometry in mm, margins, dpi — seeded with the common
Brother DK sizes, the QL family being what prompted the feature), TEMPLATES are
one design each, of two kinds: the QSO label glued on the card, whose repeating
table carries several contacts of the same station, and the address label for
the envelope, whose lines collapse when a variable is empty.

Everything is measured in millimetres — labels are sold in mm, and an operator
lining a design up against a physical sticker thinks in mm; pixels exist only in
the renderer, at the stock's dpi. The canvas renderer is shared between the
editor's preview and the future print path, so there is no second
implementation for the preview to disagree with.

The preview is fed with the log's latest contacts rather than lorem ipsum: real
data shows a too-narrow column immediately.

Printing (PDF, one page per label at exact size) is the next module; nothing
here prints yet.

app.go

@@ -44,6 +44,7 @@ import (
 	"hamlog/internal/gridcache"
 	"hamlog/internal/integrations/udp"
 	"hamlog/internal/kpa"
+	"hamlog/internal/labels"
 	"hamlog/internal/lookup"
 	"hamlog/internal/lotwusers"
 	"hamlog/internal/netctl"
@@ -735,6 +736,7 @@ type App struct {
 	uls          *uls.Store // US callsign→county/grid (offline FCC ULS), lazily opened
 	awardRefs    *awardref.Repo
 	qslTemplates *qslcard.Repo
+	labelRepo    *labels.Repo // label designer (stocks + templates)
 	operating    *operating.Repo
 	udp          *udp.Manager
 	udpRepo      *udp.Repo
@@ -1185,6 +1187,7 @@ func (a *App) startup(ctx context.Context) {
 	}
 	a.awardRefs = awardref.NewRepo(conn)
 	a.qslTemplates = qslcard.NewRepo(conn)
+	a.labelRepo = labels.NewRepo(conn)
 	a.migrateAwardDefs()      // upgrade legacy award definitions (enable + new fields)
 	a.seedBuiltinReferences() // first-run: populate built-in award reference lists
 	a.mirrorAwards()          // keep <data>/awards/*.json in step with the database