feat(portals): add craftable portal blocks replacing hardcoded coordinates

Portal frame crafted from obsidian + ender pearl, combined with crystals
(emerald/amethyst/prismarine) to create world-specific portal blocks.
Stepping on a portal block triggers transfer. Includes resource pack with
vanilla textures, block definitions, crafting recipes, and updated script.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lobby-addon/lobby_transfer_BP/recipes/portal_frame.json

@@ -0,0 +1,25 @@
+{
+  "format_version": "1.21.40",
+  "minecraft:recipe_shaped": {
+    "description": {
+      "identifier": "silverlabs:portal_frame"
+    },
+    "tags": ["crafting_table"],
+    "pattern": [
+      " O ",
+      "OEO",
+      " O "
+    ],
+    "key": {
+      "O": { "item": "minecraft:obsidian" },
+      "E": { "item": "minecraft:ender_pearl" }
+    },
+    "unlock": [
+      { "item": "minecraft:obsidian" }
+    ],
+    "result": {
+      "item": "silverlabs:portal_frame",
+      "count": 1
+    }
+  }
+}