feat(configure): add "About mureo" menu — version + installed extension packages (#229)

[hyoshi]

Summary

Adds an "About mureo" menu to the configure dashboard (#229): the mureo logo, the installed mureo version, and every installed extension package (bridges/agency) with its version — so operators and support can see the full stack at a glance.

Design

• Server — new mureo/web/about.py: collect_about_info() resolves every distribution contributing to the entry-point groups mureo.providers, mureo.runtime_context_factory, mureo.web_extensions to {name, version} via stdlib importlib.metadata, deduplicated by distribution name and name-sorted, with mureo itself always seeded first (seed wins over entry-point duplicates). Per-group and per-entry-point fault isolation mirrors mureo/web/extensions.py — one broken plugin never breaks the endpoint. Python 3.10's EntryPoint.dist is None is handled via a packages_distributions() fallback; PackageNotFoundError (dev installs) falls back to "unknown".
• Endpoint — GET /api/about in mureo/web/handlers.py, behind the same Host-header gate (DNS-rebinding defense) as every other GET JSON route. Response: {"mureo": {"name", "version"}, "packages": [{"name", "version"}, ...]}. No secrets, no file paths.
• UI — new About tab in the dashboard nav (app.html), read-only section reusing the existing logo light/dark pair; renderAbout() in dashboard.js builds the table with textContent only (no innerHTML) and re-renders on locale change; 6 i18n keys × en/ja in i18n.json ("About mureo" / 「mureoについて」).

Because discovery is entry-point based, multi-account backends and future plugins appear automatically with no per-package wiring.

Test plan

• tests/test_web_about.py — module tests with mocked metadata surfaces (mureo always present, dedup across groups, sorted order, seed-wins, fault isolation, dist-None and PackageNotFoundError fallbacks) + asset pins (HTML tab, JS fetch, i18n keys)
• tests/test_web_handlers.py::TestServeAbout — documented JSON shape (mocked), unmocked smoke test tolerant of locally installed plugins, spoofed-Host 403
• 210 tests pass across the web test files; ruff + black clean
• CI green on the PR

Closes #229