Replace Streamlit with a polished local web interface

[brainx]

Summary

• replace the Streamlit interface and transition fallback with a responsive monochrome React/Vite app served by the loopback-only Python API
• redesign Encrypt, Decrypt, Generate keys, and Inspect key as focused workflows with responsive navigation, progressive technical details, keyboard focus, reduced-motion support, and explicit capability states
• harden sensitive workflow lifecycles with request cancellation, fail-closed validation, safe error categories, password clearing, and download-specific recovery
• update packaging, lock files, CI, documentation, and screenshots for the single supported local web interface
• preserve authenticated decrypt-only compatibility for supported older encrypted artifacts while keeping new encryption on format v4 with ML-KEM-768+X25519-v2

Why

The custom frontend had reached workflow parity but still carried a temporary Streamlit fallback and mixed product, state, and styling concerns. This change makes the polished local web app the only GUI, removes the transition surface, and gives security-sensitive operations clearer state and failure handling.

User impact

• ./start.sh now always launches the local ASGI web app.
• LEGACY_STREAMLIT=1, pqc_app.py, the Streamlit configuration, and Streamlit runtime dependencies are removed.
• Existing supported encrypted artifacts remain decryptable; older public keys cannot be used for new encryption.
• The interface remains local-only and does not add telemetry, remote assets, browser persistence, or plaintext previews.

Verification

• ./test.sh — 173 passed, 15 skipped where the host-native liboqs backend was unavailable
• npm ci — clean install completed
• npm run check — TypeScript passed; 3 API-client tests and 65 component/unit tests passed
• npm run build — production build passed; 40 modules transformed
• npm run ui-smoke — Encrypt, Decrypt, Generate, Inspect, failure states, automated accessibility checks, and 390/768/1024/1440 px layouts passed
• disposable Linux/liboqs validation — 188 tests passed, including direct KEM and complete CLI encrypt/decrypt round trips
• git diff --check origin/main...HEAD — passed

Runtime change

This intentionally removes the former Streamlit recovery path. The loopback ASGI service and React/Vite client are now the sole GUI runtime.