fix(addons): correct recipe unlock shape and block format_version

Follow-up to 4324882. The { "context": "always_unlocked" } form triggered
"malformed unlocking context" on 1.26-series Bedrock (AlwaysUnlocked pascal
form fared no better). Empty-array form is the universally accepted "recipe
always known" shape. This unblocks mc-lobby which was crashing on boot.

Also reverts block format_version on mailbox.json + smart_crafting_table.json
to 1.21.0 — every other working block in the repo uses that, so the prior
1.21.60 choice was unnecessary.

- 19 dragon + portal recipes (spark_pet_BP, lobby_transfer_BP): unlock -> []
- 7 other recipes touching unlock { context: ... }: same
- 2 block files: format_version 1.21.60 -> 1.21.0

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

home-sign-addon/home_sign_BP/recipes/home_sign.json

@@ -4,11 +4,17 @@
     "description": {
       "identifier": "silverlabs:home_sign_recipe"
     },
-    "tags": ["crafting_table"],
-    "unlock": { "context": "AlwaysUnlocked" },
+    "tags": [
+      "crafting_table"
+    ],
+    "unlock": [],
     "ingredients": [
-      { "item": "minecraft:oak_sign" },
-      { "item": "minecraft:red_dye" }
+      {
+        "item": "minecraft:oak_sign"
+      },
+      {
+        "item": "minecraft:red_dye"
+      }
     ],
     "result": {
       "item": "silverlabs:home_sign",