Add desync tracking metrics (#2707)

## Description:

- Added observable gauge to track total number of client desyncs
detected across game servers
- Tracks desyncs at the individual client level (counts each out-of-sync
client)
- Exposes metric as `openfront.desyncs.gauge` for monitoring

## 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
- [ ] 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:

evan

src/server/GameServer.ts

@@ -69,6 +69,8 @@ export class GameServer {
     { winner: ClientSendWinnerMessage; ips: Set<string> }
   > = new Map();
 
+  public desyncCount = 0;
+
   constructor(
     public readonly id: string,
     readonly log_: Logger,
@@ -778,6 +780,8 @@ export class GameServer {
     const { mostCommonHash, outOfSyncClients } =
       this.findOutOfSyncClients(lastHashTurn);
 
+    this.desyncCount += outOfSyncClients.length;
+
     if (outOfSyncClients.length === 0) {
       this.turns[lastHashTurn].hash = mostCommonHash;
       return;