feat: add operator night mode

[shaypal5]

Summary

• Add a persisted day/night mode preference for the operator dashboard.
• Add a topbar icon toggle with accessible labels for switching modes.
• Apply night-mode colors after deployment branding so branded builds can still be switched to a dark UI.

Validation

• npm run check && npm run test && npm run build && git diff --check
• Local Playwright smoke on http://127.0.0.1:4177/?demo=1 verified day -> night -> day toggle state, CSS variable change, accessible labels, and localStorage persistence.

Follow-up

• No repo milestone exists to assign.