feat(play-standalone): Add PEER_URL env variable (#302)

## Description

Adds PEER_URL env variable for setting peer URL (query param still takes
priority if set).

- Useful for self-hosters
- Was a pain to figure out



<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- New Features
- Support configuring the peer server URL via an environment variable,
with automatic fallback to the URL parameter or a default.
- Server-provided configuration is securely passed to the client to
simplify deployment setup.

- Chores
- Excluded common build artifacts and IDE directories from container
contexts to reduce image size and speed up builds.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: DatCaptainHorse <DatCaptainHorse@users.noreply.github.com>

packages/play-standalone/astro.config.mjs

@@ -1,5 +1,5 @@
 // @ts-check
-import { defineConfig } from "astro/config";
+import { defineConfig, envField } from "astro/config";
 import node from "@astrojs/node";
 
 // https://astro.build/config
@@ -12,4 +12,9 @@ export default defineConfig({
     "host": "0.0.0.0",
     "port": 3000,
   },
+  env: {
+    schema: {
+      PEER_URL: envField.string({ context: "server", access: "secret", optional: true }),
+    }
+  }
 });