## Description:

When random spawn mode is active, players are supposed to receive
randomly chosen spawns rather than choosing their own. However,
`SpawnExecution.getSpawn()` checks `center !== undefined` first, which
means if a player manually injects coordinates into the spawn intent
(bypassing the client-side UI guard), the random selection logic is
completely bypassed and the player gets their chosen coordinates.

This was fully exploitable in singleplayer (where no pre-created
`SpawnExecution` objects exist) and was a defense-in-depth gap in
multiplayer (relying on execution order of pre-created spawns to block
it via the `hasSpawned()` guard).

The fix forces `center` to `undefined` in `getSpawn()` when random
spawns are enabled, ensuring the random selection code path is always
taken regardless of what the client sends.

## Changes:
- `src/core/execution/SpawnExecution.ts`: Pass `undefined` to
`getSpawn()` when `isRandomSpawn()` is true, ignoring any
client-specified tile
- `tests/core/execution/SpawnExecution.test.ts`: Added test verifying
that a client-specified tile is ignored when random spawn is enabled

## Please complete the following:

- [X] I have added screenshots for all UI updates
- [X] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [X] I have added relevant tests to the test directory

## Please put your Discord username so you can be contacted if a bug or
regression is found:

FloPinguin