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/manifest.json

@@ -17,7 +17,7 @@
       "type": "script",
       "language": "javascript",
       "uuid": "d7a4b9c1-2e3f-4a5b-8c6d-1f2e3d4c5b62",
-      "version": [1, 0, 0],
+      "version": [1, 1, 0],
       "entry": "scripts/main.js"
     }
   ],