Fix: skip session delta entries with no tank definition or average

The /session/ page crashed when a tracked player's tank stats included
a tank with no matching tank definition or no average stats (WN8
baseline). The delta memo dereferenced `tank.id` on a possibly missing
definition and forced `average?.mu!` through as non-null, so a missing
average threw inside `compositeStats`/`calculateWN8` uncaught, breaking
the page on every load while the bad state persisted in localStorage.

Skip those entries instead of crashing: entries with no tank
definition or no average `mu` are dropped from the delta via
`flatMap`, so the rest of the session table still renders.

packages/website/src/pages/[...chunk_default]/[...locale]/session/_index.tsx

@@ -90,15 +90,19 @@ function Content() {
       session.tracking && tankStatsB
         ? deltaTankStats(session.player.stats, tankStatsB)
             .sort((a, b) => b.last_battle_time - a.last_battle_time)
-            .map((entry) => {
+            .flatMap((entry) => {
               const tank = tankDefinitions.tanks[entry.tank_id];
+              if (!tank) return [];
+
               const average = averageDefinitions.averages[tank.id];
+              if (!average?.mu) return [];
+
               const composite = compositeStats(
                 { ...entry.all, battle_life_time: entry.battle_life_time },
-                average?.mu!,
+                average.mu,
               );
 
-              return { tank, composite };
+              return [{ tank, composite }];
             })
         : undefined,
     [session.tracking && session.player, tankStatsB],