Add testnet warning (#880)

* Add testnet warning

* Add watermarks to posts

* Call the test environment the Sandbox

src/state/models/session.ts

@@ -10,6 +10,7 @@ import {isObj, hasProp} from 'lib/type-guards'
 import {networkRetry} from 'lib/async/retry'
 import {z} from 'zod'
 import {RootStoreModel} from './root-store'
+import {IS_PROD} from 'lib/constants'
 
 export type ServiceDescription = DescribeServer.OutputSchema
 
@@ -104,6 +105,13 @@ export class SessionModel {
     return this.accounts.filter(acct => acct.did !== this.data?.did)
   }
 
+  get isSandbox() {
+    if (!this.data) {
+      return false
+    }
+    return !IS_PROD(this.data.service)
+  }
+
   serialize(): unknown {
     return {
       data: this.data,