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>

package.json

@@ -13,7 +13,10 @@
     "seed": "pnpm --filter server seed"
   },
   "devDependencies": {
+    "@typescript-eslint/eslint-plugin": "^8.56.1",
+    "@typescript-eslint/parser": "^8.56.1",
     "concurrently": "^9.1.2",
+    "eslint": "^10.0.3",
     "typescript": "^5.7.3",
     "vitest": "^3.1.1"
   }