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_RP/textures/item_texture.json

@@ -0,0 +1,12 @@
+{
+  "resource_pack_name": "postal_service_RP",
+  "texture_name": "atlas.items",
+  "texture_data": {
+    "mailbox_icon": {
+      "textures": "textures/items/mailbox"
+    },
+    "post_office_icon": {
+      "textures": "textures/items/post_office"
+    }
+  }
+}