feat: Add 'system' theme

[cpAdm]

• Document theme (dark/light) listens to system's theme if user's theme is 'system'
• Changed all darkmode checkboxes to select inputs
• Added some stricter typing to settingsView.tsx and uiUtils.ts::Settings
• Moved the HeaderView settings dialog outside the button, this was intercepting events for the select
• Changed existing dark mode test, and added another for system theme change

Inspector	HTML reporter	Trace viewer	UI mode

Closes: #37874

[github-actions]

Test results for "MCP"

2601 passed, 116 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

4 flaky

⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:1079 › cli codegen › should not throw csp directive violation errors `@firefox-ubuntu-22.04-node18`
⚠️ [firefox-page] › page/page-event-request.spec.ts:182 › should return response body when Cross-Origin-Opener-Policy is set `@firefox-ubuntu-22.04-node18`
⚠️ [playwright-test] › ui-mode-test-output.spec.ts:117 › should collapse repeated console messages for test `@ubuntu-latest-node24-2`
⚠️ [playwright-test] › ui-mode-test-attachments.spec.ts:61 › should contain binary attachment `@windows-latest-node18-2`

39424 passed, 784 skipped

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Merge workflow run.