chore: add ESLint with no-floating-promises and no-console rules

Sets up ESLint 10 flat config with @typescript-eslint/no-floating-promises (error)
and no-console (warn, allow warn/error). Fixes all floating promise errors in
server routes, orchestrator reader cleanup, and frontend composables with void operator.
Game engine files get warning-level for intentional fire-and-forget async.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/routes/fights.ts

@@ -391,7 +391,7 @@ fightsRouter.get('/:id/stream', (c) => {
     })
 
     const cleanup = fightEvents.on(fightId, (event) => {
-      stream.writeSSE({
+      void stream.writeSSE({
         event: event.type,
         data: JSON.stringify({ ...event.data, spectators: spectatorCounts.get(fightId) || 0 }),
       })
@@ -399,7 +399,7 @@ fightsRouter.get('/:id/stream', (c) => {
 
     const cleanupGlobal = fightEvents.onAll((event) => {
       if (event.fightId === fightId && event.type === 'fight_end') {
-        stream.writeSSE({
+        void stream.writeSSE({
           event: 'fight_end',
           data: JSON.stringify({ ...event.data, spectators: spectatorCounts.get(fightId) || 0 }),
         })