feat: enable Dashboard by default without blocking Server startup

[Zxf-xufeng]

Closes #1290

Rationale for this change

PowerContext Server did not enable Dashboard by default, and Dashboard initialization could make startup failures affect core Server availability. Users also lacked a clear startup URL.

What changes are included in this PR?

• Enable Dashboard by default when Server starts.
• Print the Dashboard URL (/) on startup.
• Allow Dashboard to run without configured scopes and without bearer auth when Server auth is disabled.
• Make Dashboard initialization fail open: print/log the direct reason and continue HTTP API, MCP, and health endpoints.
• Add an explicit environment switch: POWERCONTEXT_SERVER_DASHBOARD_ENABLED=false.
• Update English and Chinese documentation and regression tests.
• Dashboard, HTTP API, and MCP share the same listener and port; automatic port fallback is out of scope.

Are there any user-facing changes?

Yes. Dashboard is available by default at http://127.0.0.1:8000/ (assuming the default host and port), and startup prints the URL. Dashboard can be disabled with the environment variable above. A Dashboard startup failure no longer prevents the core Server from starting. No breaking API changes.

How was this change tested?

• env POWERCONTEXT_SERVER_AUTH_ENABLED=false POWERCONTEXT_SERVER_DASHBOARD_ENABLED=true .venv/bin/pytest tests/test_dashboard.py tests/test_server.py tests/test_cli.py -q (50 passed)
• .venv/bin/ty check src/powercontext/server src/powercontext/cli passed
• .venv/bin/zensical build -s passed
• Ruff lint and format checks passed.
• JavaScript syntax checks passed.
• git diff --check passed.

AI usage statement

Built with OpenAI Codex (GPT-5-based coding agent), used for implementation, tests, and documentation updates.

[Teingi]

LGTM