Do not hardcode 32100 port (#1969)

Fixes #1949 

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## Summary by cubic
Make app ports dynamic instead of hardcoded 32100 to prevent conflicts
and keep local runs, Docker, and env URLs in sync. Ports now derive from
appId using a base of 32100.

- **Bug Fixes**
  - Added getAppPort(appId) = 32100 + (appId % 10_000).
- Used the dynamic port in NEXT_PUBLIC_SERVER_URL, start commands,
Docker -p mapping, and cleanUpPort.
- Updated getCommand to accept appId and generate a per-app default
start command.

<sup>Written for commit 46ac310c762fd4044c35bc59264122234ed19bbf.
Summary will update automatically on new commits.</sup>

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shared/ports.ts

@@ -0,0 +1,7 @@
+/**
+ * Calculate the port for a given app based on its ID.
+ * Uses a base port of 32100 and offsets by appId % 10_000.
+ */
+export function getAppPort(appId: number): number {
+  return 32100 + (appId % 10_000);
+}