fix(embed): use family-agnostic localhost probe and port helpers (#3527)

[chethanuk]

Description

hindsight-embed ui start and ui status report failure when the control plane binds IPv6 ([::1]) under --hostname localhost or in dual-stack environments. The UI health probe previously hardcoded IPv4 127.0.0.1, which is refused when Next.js binds IPv6-only ::1.

Root Cause

• daemon_embed_manager.py:is_ui_running hardcoded 127.0.0.1 for /api/health.
• _is_port_in_use used an AF_INET-only socket, treating live IPv6 listeners as free ports.
• _find_pid_on_port filtered Windows netstat output for 127.0.0.1:{port} substring.

Fix

1. get_ui_url: Default host fallback to localhost instead of 0.0.0.0.
2. is_ui_running: Remove hardcoded 127.0.0.1 override; probe localhost so httpx resolves both IPv4 and IPv6 families via getaddrinfo.
3. _is_port_in_use: Use stdlib socket.create_connection(("localhost", port), timeout=1) for dual-stack support.
4. _find_pid_on_port: Replace substring check with bounds-checked netstat column parsing.
5. Printed URLs: Update CLI messages to http://localhost:{port}.

Testing

Added hindsight-embed/tests/test_ui_health_probe.py covering:

• IPv6-only (::1) health probe response (failing before fix, passing now)
• IPv4-only (127.0.0.1) health probe response
• Closed port fail-fast (<5s)
• Windows netstat parsing for [::1]:port and 127.0.0.1:port

All 169 unit & integration tests pass cleanly:

============================= 169 passed in 22.22s =============================

Fixes #3527