fix(webapp): keep playground chat requests same-origin

[isshaddad]

Problem

The agent playground chat builds its realtime transport baseURL from apiOrigin, but points it at a same-origin /resources/... dashboard route. When API_ORIGIN differs from APP_ORIGIN, the in/append POST goes cross-origin, fails the CORS preflight, and messages never reach the agent ("Failed to fetch").

It only reproduces where the two origins differ — not locally, where both default to localhost:3030.

Fixes #4149.

Fix

Build the base URL from window.location.origin (falling back to apiOrigin on SSR), so realtime traffic stays same-origin — the same approach AgentView.tsx already uses.

Testing

Typecheck passes. The CORS path only manifests when API_ORIGIN != APP_ORIGIN, so verify on test-cloud (can't reproduce locally).

[changeset-bot]

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Latest commit: 9ad4a73

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[coderabbitai]

Walkthrough

This change modifies the playground chat resource base URL construction to prefer same-origin requests. In the browser, an origin variable now uses window.location.origin, falling back to apiOrigin when unavailable, and playgroundBaseURL is rebuilt using this origin instead of always defaulting to apiOrigin. A changelog entry was added documenting the fix for a "Failed to fetch" error previously encountered when sending messages in the agent playground chat in certain environments.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The code change matches #4149 by switching the playground chat base URL to same-origin, which addresses the CORS failure.
Out of Scope Changes check	✅ Passed	The changes stay focused on the playground chat URL fix and the accompanying changelog note.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title is concise and accurately summarizes the main change: keeping playground chat requests same-origin.
Description check	✅ Passed	The description clearly covers the problem, fix, and testing, though it does not fully follow the template sections.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/playground-chat-same-origin-base-url

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.