Enable strictNullChecks, eqeqeq (#436)

## Description:

Improve type safety and runtime correctness by:
1. Enabling TypeScript's
[strictNullChecks](https://www.typescriptlang.org/tsconfig/#strictNullChecks)
compiler option.
2. Replacing all loose equality operators (`==` and `!=`) with strict
equality operators (`===` and `!==`).
3. Cleaning up of type declarations, null handling logic, and equality
expressions throughout the project.

Currently, the code allows implicit assumptions that `null` and
`undefined` are interchangeable, and relies on type-coercing equality
checks that can introduce subtle bugs. These practices make it difficult
to reason about when values may be absent and hinder the effectiveness
of static analysis.

Migrating to strict null checks and enforcing strict equality
comparisons will clarify intent, reduce bugs, and make the codebase
safer and easier to maintain.

Fixes #466 

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

---------

Co-authored-by: Scott Anderson <662325+scottanderson@users.noreply.github.com>
Co-authored-by: evanpelle <openfrontio@gmail.com>

src/client/ClientGameRunner.ts

@@ -63,7 +63,7 @@ export function joinLobby(
   );
 
   const userSettings: UserSettings = new UserSettings();
-  startGame(lobbyConfig.gameID, lobbyConfig.gameStartInfo?.config);
+  startGame(lobbyConfig.gameID, lobbyConfig.gameStartInfo?.config ?? {});
 
   const transport = new Transport(lobbyConfig, eventBus);
 
@@ -74,12 +74,12 @@ export function joinLobby(
   let terrainLoad: Promise<TerrainMapData> | null = null;
 
   const onmessage = (message: ServerMessage) => {
-    if (message.type == "prestart") {
+    if (message.type === "prestart") {
       consolex.log(`lobby: game prestarting: ${JSON.stringify(message)}`);
       terrainLoad = loadTerrainMap(message.gameMap);
       onPrestart();
     }
-    if (message.type == "start") {
+    if (message.type === "start") {
       // Trigger prestart for singleplayer games
       onPrestart();
       consolex.log(`lobby: game started: ${JSON.stringify(message, null, 2)}`);
@@ -109,10 +109,13 @@ export async function createClientGame(
   userSettings: UserSettings,
   terrainLoad: Promise<TerrainMapData> | null,
 ): Promise<ClientGameRunner> {
+  if (lobbyConfig.gameStartInfo === undefined) {
+    throw new Error("missing gameStartInfo");
+  }
   const config = await getConfig(
     lobbyConfig.gameStartInfo.config,
     userSettings,
-    lobbyConfig.gameRecord != null,
+    lobbyConfig.gameRecord !== undefined,
   );
   let gameMap: TerrainMapData | null = null;
 
@@ -160,7 +163,7 @@ export async function createClientGame(
 }
 
 export class ClientGameRunner {
-  private myPlayer: PlayerView;
+  private myPlayer: PlayerView | null = null;
   private isActive = false;
 
   private turnsSeen = 0;
@@ -193,7 +196,7 @@ export class ClientGameRunner {
       },
     ];
     let winner: ClientID | Team | null = null;
-    if (update.winnerType == "player") {
+    if (update.winnerType === "player") {
       winner = this.gameView
         .playerBySmallID(update.winner as number)
         .clientID();
@@ -201,6 +204,9 @@ export class ClientGameRunner {
       winner = update.winner as Team;
     }
 
+    if (this.lobby.gameStartInfo === undefined) {
+      throw new Error("missing gameStartInfo");
+    }
     const record = createGameRecord(
       this.lobby.gameStartInfo.gameID,
       this.lobby.gameStartInfo,
@@ -233,10 +239,13 @@ export class ClientGameRunner {
     this.renderer.initialize();
     this.input.initialize();
     this.worker.start((gu: GameUpdateViewData | ErrorUpdate) => {
+      if (this.lobby.gameStartInfo === undefined) {
+        throw new Error("missing gameStartInfo");
+      }
       if ("errMsg" in gu) {
         showErrorModal(
           gu.errMsg,
-          gu.stack,
+          gu.stack ?? "missing",
           this.lobby.gameStartInfo.gameID,
           this.lobby.clientID,
         );
@@ -268,7 +277,7 @@ export class ClientGameRunner {
     };
     const onmessage = (message: ServerMessage) => {
       this.lastMessageTime = Date.now();
-      if (message.type == "start") {
+      if (message.type === "start") {
         this.hasJoined = true;
         consolex.log("starting game!");
         for (const turn of message.turns) {
@@ -286,7 +295,10 @@ export class ClientGameRunner {
           this.turnsSeen++;
         }
       }
-      if (message.type == "desync") {
+      if (message.type === "desync") {
+        if (this.lobby.gameStartInfo === undefined) {
+          throw new Error("missing gameStartInfo");
+        }
         showErrorModal(
           `desync from server: ${JSON.stringify(message)}`,
           "",
@@ -296,12 +308,12 @@ export class ClientGameRunner {
           "You are desynced from other players. What you see might differ from other players.",
         );
       }
-      if (message.type == "turn") {
+      if (message.type === "turn") {
         if (!this.hasJoined) {
           this.transport.joinGame(0);
           return;
         }
-        if (this.turnsSeen != message.turn.turnNumber) {
+        if (this.turnsSeen !== message.turn.turnNumber) {
           consolex.error(
             `got wrong turn have turns ${this.turnsSeen}, received turn ${message.turn.turnNumber}`,
           );
@@ -348,17 +360,17 @@ export class ClientGameRunner {
     if (this.gameView.inSpawnPhase()) {
       return;
     }
-    if (this.myPlayer == null) {
-      this.myPlayer = this.gameView.playerByClientID(this.lobby.clientID);
-      if (this.myPlayer == null) {
-        return;
-      }
+    if (this.myPlayer === null) {
+      const myPlayer = this.gameView.playerByClientID(this.lobby.clientID);
+      if (myPlayer === null) return;
+      this.myPlayer = myPlayer;
     }
     this.myPlayer.actions(tile).then((actions) => {
+      if (this.myPlayer === null) return;
       const bu = actions.buildableUnits.find(
-        (bu) => bu.type == UnitType.TransportShip,
+        (bu) => bu.type === UnitType.TransportShip,
       );
-      if (bu == null) {
+      if (bu === undefined) {
         console.warn(`no transport ship buildable units`);
         return;
       }
@@ -377,7 +389,8 @@ export class ClientGameRunner {
         this.myPlayer
           .bestTransportShipSpawn(this.gameView.ref(cell.x, cell.y))
           .then((spawn: number | false) => {
-            let spawnCell = null;
+            if (this.myPlayer === null) throw new Error("not initialized");
+            let spawnCell: Cell | null = null;
             if (spawn !== false) {
               spawnCell = new Cell(
                 this.gameView.x(spawn),