feat(pocket-base): allow return portal relocation

design/user-stories/us-016-build-and-use-pocket-base-portals.md

@@ -21,7 +21,12 @@ As a **Pocket Base owner**, I want to connect my normal base to my Pocket Base w
 - [x] Any player can use an active entrance without a separate visitor setting, warm-up, or cooldown.
 - [x] Visitors can build, break blocks, open containers, and otherwise interact normally inside the Pocket Base.
 - [x] The generated return portal sends players safely to the owner's active entrance.
-- [x] Return travel falls back to the server spawn when the owner has no valid active entrance.
+- [x] Only the owner can activate a complete portal frame inside their own Pocket Base with flint and steel.
+- [x] Every required return-frame block must be within the owner's unlocked Pocket Base boundary.
+- [x] Each Pocket Base has at most one active return portal, and activating another valid frame moves the active return location without removing the old frame.
+- [x] Relocated return portals persist through restarts, while existing Pocket Bases use the generated portal as their default.
+- [x] Breaking the active return frame disables return travel through it.
+- [x] Return travel falls back to the server spawn when the owner has no valid normal-world entrance.
 - [x] Falling below Y -64 performs the same safe return without void damage.
 - [x] Players who disconnect inside a Pocket Base remain there when they reconnect.
 - [x] Moving the normal base deactivates its entrance while preserving the Pocket Base world and its contents.