fix(Web): Dark Mode isn't friendly to find

[cocojojo5213]

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Changes

Closes #7702

Adds an always-available theme selector to the app header so users can switch theme without leaving their current page. The selector is shown in the existing desktop top nav and as a compact mobile header action.

• Adds Light, Dark, and System theme choices without using an "auto" label.
• Keeps the account settings appearance control in sync with the header selector.
• Updates other open tabs immediately via local storage events while preserving the legacy dark_mode value for compatibility.

Review effort: 3/5

How did you test this code?

• npx eslint --fix web/project/darkMode.ts web/project/darkMode.test.ts web/components/DarkModeSwitch.tsx web/components/navigation/Nav.tsx web/components/navigation/navbars/TopNavbar.tsx common/stores/account-store.js
• npm run test:unit -- --runTestsByPath web/project/darkMode.test.ts
• npm run bundle
• git diff --check

I also checked the touched files against tsc; the full repository typecheck currently reports existing errors outside this change.

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[cocojojo5213]

Validation note: pre-commit.ci passes and I ran the focused frontend checks listed in the PR description locally. The current Vercel failures say "Authorization required to deploy", and the GitHub Actions frontend workflow is waiting for maintainer approval for this fork PR.