fix(embed): probe both loopback families in UI health check

[NovaLux12]

Closes #3527

Problem

hindsight-embed ui start / ui status report the Control Plane UI as not
running even when it is up: is_ui_running() health-checks a hardcoded
http://127.0.0.1:<port>/api/health URL, but when the UI binds only the IPv6
loopback (e.g. Next.js started with --hostname localhost, the documented
#1926 workaround) the probe is refused. Every command that depends on the
check — ui start's ready-wait, ui status — reports failure, and ui start
times out after 30s while the UI is actually serving on [::1].

Change

• DaemonEmbedManager.is_ui_running() now probes both loopback families,
  127.0.0.1 then ::1, returning True on the first healthy response.
• get_ui_url() brackets IPv6 literals (::1 → [::1]) so the probe URL —
  and any display of an IPv6 hostname — is a valid URL.

Verification

• New regression tests in hindsight-embed/tests/test_embed_manager.py:
  • UI bound to [::1] only is detected as running (asserts the exact probe
    URLs, including the bracketed IPv6 form);
  • IPv4-loopback success short-circuits without probing IPv6;
  • both loopback probes refused → False;
  • get_ui_url("::1") → http://[::1]:<port>.
• uv run pytest tests/ in hindsight-embed: 162 passed; the 3 failures
  (test_profile_daemon_config.py) are pre-existing and environmental —
  they fail identically on a clean main checkout because the venv lacks
  sentence_transformers, which flips _find_api_command to the uvx
  fallback. Unrelated to this change.
• ruff check and ruff format --check pass on both changed files.

[NovaLux12]

Duplicate of #3529 (opened 4 minutes earlier by @chethanuk) — closing mine to keep the issue to a single PR. Mine took a narrower approach (explicit 127.0.0.1 then [::1] probes with bracketed IPv6 URLs in is_ui_running, +98/-9 over 2 files, 162 embed tests passing); #3529 is broader (family-agnostic localhost probe, netstat parsing, _is_port_in_use). Happy to defer to #3529 or help consolidate if the maintainers prefer either variant.