Auto-detect STREAM endpoint and remove admin input

[soyuz-dev]

Motivation

• The admin panel previously required a manually-entered STREAM sender endpoint stored in settings, which is error-prone and redundant when a sensible default can be derived from the app environment or host.
• The change aims to simplify admin UX and ensure the payout flow always has a valid STREAM endpoint without manual configuration.

Description

• Removed the persisted streamEndpoint field from AdminSettings and removed the editable STREAM endpoint input in the Admin UI in webApp/src/components/App/App.tsx.
• Added detectStreamEndpoint() which first uses VITE_STREAM_SENDER_ENDPOINT if present, otherwise derives http(s)://<host>:3000/stream/send (with a small special-case when the current origin is already on port 3000).
• Updated createInterledgerClient() and payout flow to use the auto-detected endpoint via detectStreamEndpoint() instead of reading it from admin settings, and removed the admin-side validation error that required a manually-entered endpoint.
• Replaced the admin form input with informational text that displays the auto-detected endpoint to admins.

Testing

• Ran npm --prefix webApp run build which completed successfully and produced a production build.
• Launched the local dev server and exercised the Admin panel; a Playwright run using Firefox loaded the app, logged into the Admin panel, and captured a screenshot showing the auto-detected endpoint (artifact generated).
• A Playwright Chromium run crashed in this environment with a browser SIGSEGV, which is an environment-level failure and not related to the code changes, while the Firefox-based run succeeded.

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Codex Task