Fix consent theme initialization race

[Cedric Vidal (cedricvidal)]

Summary

Fix the post-merge Pages failure from
run 38772995219.

The consent UI theme observer now ignores writes that do not change the
resolved theme. This prevents a redundant initial dark application when
Starlight and Mermaid initialize concurrently. The browser regression test
now verifies same-theme writes are ignored and real light/dark transitions
are applied.

Demos

N/A — website initialization and test-stability fix.

Before

N/A

After

N/A

Testing

• cd website && SITE=https://microsoft.github.io BASE_PATH=/scope pnpm run build
  — passed
• cd website && pnpm exec playwright test tests/analytics.spec.ts --grep 'consent UI follows Starlight theme changes' — passed, 4 tests
• cd website && pnpm exec playwright test — passed, 68 tests

Documentation and compatibility

None. No API, CLI, or deployment compatibility impact.

Checklist

• If Portal features changed, keep CLI capabilities in sync. — N/A; no Portal feature change.
• If Portal components changed, update their Storybook stories. — N/A; no Portal component change.
• If database changes require a migration, include up() / down() and keep it CosmosDB-compatible. — N/A; no database change.
• If dependencies changed, update the lockfile and regenerate NOTICE / NOTICE-REVIEW.txt with pnpm notice as needed. — N/A; no dependency change.
• Video showing the behavior before the suggested change — N/A; initialization race covered by automated regression test.
• Video showing the behavior after the suggested change — N/A; initialization race covered by automated regression test.