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/engine/orchestrator.ts

@@ -114,13 +114,13 @@ async function readLimitedBody(res: Response, maxBytes: number): Promise<string>
       if (done) break
       totalBytes += value.byteLength
       if (totalBytes > maxBytes) {
-        reader.cancel()
+        void reader.cancel()
         throw new Error(`Response body exceeds ${maxBytes} bytes`)
       }
       chunks.push(value)
     }
   } catch (err) {
-    reader.cancel()
+    void reader.cancel()
     throw err
   }
   const combined = new Uint8Array(totalBytes)