feat: remove spawn timer on singleplayer (#3199)

Resolves #1041 

## Description:

Remove the singleplayer spawn countdown so the game starts when the
player spawns, spawn nations immediately after player spawn, and align
game timer/max-timer timing with the new start point.

Added a singleplayer regression test for spawn-immunity timing
(GameImpl.test.ts) and updated spawn-phase loop tests to use gameType:
GameType.Public where singleplayer behavior is not under test (e.g.
MIRV/AI/Spawn/WinCheck-related suites), eliminating inSpawnPhase()
timeout hangs after the new singleplayer start logic.


https://github.com/user-attachments/assets/c07a585f-1153-490e-88ca-a91fc7ae5756

## 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
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

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

tests/core/executions/SAMLauncherExecution.test.ts

@@ -78,10 +78,6 @@ describe("SAM", () => {
       ),
     );
 
-    while (game.inSpawnPhase()) {
-      game.executeNextTick();
-    }
-
     attacker = game.player("attacker_id");
     defender = game.player("defender_id");
     middle_defender = game.player("middle_defender_id");