Replace hardcoded port 3000 with dynamic port in dev URLs

[p-jackson]

Part of #109963

Proposed Changes

• Add {{key}} template support to config values in create-calypso-config, resolved at config creation time
• Config JSON files (development.json, dashboard-development.json) use {{port}} in URL values (wpcom_url, logout_url) so they reflect the actual server port
• Client code uses config('port') for dev URLs in dashboard routing, jetpack-connect, and checkout
• CORS allowlists in wpcom-proxy-request and calypso-url match localhost origins on any port by stripping the port and comparing against known dev origins
• Export resolveTemplates from create-calypso-config for server-side use on clientData (which must remain a plain object)

Why are these changes being made?

When running Calypso on a non-default port (e.g. via PORT=4321 or cmux/conductor), many URLs were hardcoded to :3000 — the Reader link in the masterbar, dashboard cross-origin links, CORS allowlists, etc. This makes the port fully dynamic so all dev URLs match the actual server port.

Testing Instructions

• Run PORT=4321 yarn start (or any non-3000 port)
• Verify the Reader link in the masterbar points to :4321
• Verify window.configData.wpcom_url contains the correct port
• Verify no :3000 appears in the HTML source: curl -s http://calypso.localhost:4321 | grep -c ':3000' should be 0
• Verify dashboard links work on http://my.localhost:4321

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175649

Jetpack Cloud Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175649&env=jetpack

Automattic for Agencies Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175649&env=a8c-for-agencies

Dashboard Live (dotcom) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175649&env=dashboard

Dashboard Live (CIAB) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175649&env=dashboard-ciab

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• agents-manager
• blaze-dashboard
• happy-blocks
• help-center
• notifications
• odyssey-stats
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug dot-env-port-dynamic-port on your sandbox.