[Dashboard] Implement sign-out functionality with OAuth2 support

[ibpark-moreh]

This PR adds a 'Sign out' button to the SkyPilot dashboard.

SkyPilot supports two authentication methods: OAuth2 and HTTP Basic Auth. This PR implements conditional behavior for the sign-out button based on the active authentication method.

• When OAuth2 is enabled:

  • The 'Sign out' button is displayed on the dashboard.
  • When clicked, it calls the /oauth2/sign_out endpoint provided by oauth2-proxy to securely sign the user out.

• When HTTP Basic Auth is enabled:

  • The 'Sign out' button is hidden.
  • This is because HTTP Basic Auth credentials are managed directly by the browser, and the web application (dashboard) cannot programmatically clear them to force a sign-out. Hiding the button prevents user confusion.

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)