feat(subs): add a "how many periods" count that drives the end date

When subscriptions moved to the plan model the span became start + end dates,
which lost the simple "renew for N months/weeks/days" input — the operator had
to hand-compute the end date. (quantity is CARS, a separate axis, not periods.)

Add a count field to the sell form: the operator types e.g. 3, and validTo is
auto-derived as validFrom + count × the plan's period (day/week/month), with the
same month-overflow clamp the server uses (Jan 31 +1mo → Feb 28) so the preview
matches what's stored + charged. The end-date field stays directly editable for
an irregular span (the hotel checkout case), and editing it isn't overwritten by
the count effect. The count row shows the plan's unit ("× month").

Build+lint 12/12 (i18n parity).

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/web/src/lib/i18n/en.ts

@@ -389,6 +389,7 @@ export const en: Catalog = {
     plan: "Plan",
     quantity: "Cars",
     quantityHint: "cars covered by this subscription (price ×N)",
+    count: "How many",
     planNone: "— comp / no charge —",
     planNoneAvail: "No plans defined — an admin must create one first.",
     quoting: "pricing…",

apps/web/src/lib/i18n/sq.ts

@@ -400,6 +400,7 @@ export const sq = {
     plan: "Plani",
     quantity: "Makina",
     quantityHint: "makina të mbuluara nga ky abonim (çmimi ×N)",
+    count: "Sa",
     planNone: "— pa pagesë / falas —",
     planNoneAvail: "Asnjë plan i përcaktuar — admini duhet të krijojë një të parin.",
     quoting: "duke llogaritur…",