feat(gateway): add polished web portal with login and dashboard

[chrystianmartins]

Motivation

• Provide a lightweight, attractive project portal for the gateway so the service exposes a human-friendly homepage and a demoable login flow.
• Give operators a simple dashboard, menus and reports UI to showcase status and KPIs without changing existing API contracts.
• Keep the change non-breaking and self-contained inside the gateway so API endpoints remain unaffected.

Description

• Added an HTML homepage served at GET / by importing HTMLResponse and registering web_portal() which returns a PORTAL_HTML constant.
• Implemented PORTAL_HTML containing a responsive, dark-themed UI with sidebar navigation, KPI cards, performance reports table and a login modal with minimal client-side JS.
• Preserved all existing endpoints and behavior; the change only adds a new route and static HTML payload.
• Added a test test_web_portal_homepage in pool-gateway/tests/test_gateway_api.py to assert the homepage returns HTTP 200 and contains the brand and login CTA.

Testing

• Ran pytest in the gateway: cd pool-gateway && pytest -q which initially failed in the CI-like environment due to missing PYTHONPATH resolution (ModuleNotFoundError importing app).
• Re-ran tests with module path set and all tests passed: cd pool-gateway && PYTHONPATH=. pytest -q resulting in 8 passed.

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Codex Task