feat(postal): multi-mailbox per player with labels, redirect flow, icon refresh

Players can now place up to 5 mailboxes, each labelled like a lodestone
waypoint. Sending mail picks recipient then mailbox; redirect collapses
one of your own mailboxes into another and removes the source.

- v1 -> v2 schema migration runs once on boot; existing claims default to
  label "Mailbox".
- Two-step send picker (skipped when recipient has only one mailbox).
- Post office root menu adds Redirect option.
- Per-entry break handling so removing one mailbox keeps the others claimed.
- minecraft:icon component + 16x16 inventory icons for both blocks.
- Refreshed pack_icon.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

postal-service-addon/postal_service_BP/blocks/post_office.json

@@ -21,7 +21,8 @@
           "texture": "post_office",
           "render_method": "opaque"
         }
-      }
+      },
+      "minecraft:icon": "post_office_icon"
     }
   }
 }