feat(cms): enable Payload localization — hu default + en (MITHOME-110)

payload.config.ts: localization: { locales: ['hu','en'], defaultLocale:
'hu', fallback: true }. URL/routing side is separate (MITHOME-114).

Proved the mechanism on a real field rather than a throwaway one:
Common.buttons.* (contact/learnMore/webmail/sendMessage) marked
localized: true — these are genuinely translatable UI labels, so this
doubles as a first, correct slice of the full MITHOME-111 retrofit
instead of being disposable test scaffolding.

Gotcha discovered and documented in the migration script: marking an
existing field `localized: true` after data was already written non-
localized makes that value unreadable via `locale: defaultLocale` (the
storage shape changed) — the migration script must be re-run so it
gets rewritten under the localized shape. This will matter again for
the full MITHOME-111/112 retrofit.

Verified: Local API round-trip (set en, defaultLocale/hu re-seeded via
re-running the migration script) — hu reads "Kapcsolatfelvétel", en
reads "Contact". Real browser: admin UI locale switcher (hu/en) in the
top bar, fields show "— hu"/"— en" per-locale labels, switching
locale swaps the visible value correctly on the Common global editor.
build/lint/tsc/test (58 passed) all clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

proto/src/globals/Common.ts

@@ -1,6 +1,14 @@
 import type { GlobalConfig } from 'payload'
 
-/** Mirrors CommonContent (proto/src/content/types.ts) — MITHOME-87. */
+/**
+ * Mirrors CommonContent (proto/src/content/types.ts) — MITHOME-87.
+ *
+ * `buttons.*` localized: true (MITHOME-110) — ez a lokalizáció-mechanizmus
+ * bizonyítéka (admin nyelvváltó + Local/REST API locale param), és egyben
+ * előremutató, helyes darabja a MITHOME-111 teljes retrofitnak: ezek
+ * ténylegesen fordítandó UI-feliratok. A többi mező itt még nem lokalizált,
+ * azt a MITHOME-111 teljes körűen elvégzi.
+ */
 export const Common: GlobalConfig = {
   slug: 'common',
   admin: {
@@ -11,10 +19,10 @@ export const Common: GlobalConfig = {
       name: 'buttons',
       type: 'group',
       fields: [
-        { name: 'contact', type: 'text', required: true },
-        { name: 'learnMore', type: 'text', required: true },
-        { name: 'webmail', type: 'text', required: true },
-        { name: 'sendMessage', type: 'text', required: true },
+        { name: 'contact', type: 'text', required: true, localized: true },
+        { name: 'learnMore', type: 'text', required: true, localized: true },
+        { name: 'webmail', type: 'text', required: true, localized: true },
+        { name: 'sendMessage', type: 'text', required: true, localized: true },
       ],
     },
     {