Dashboard: recolor the interim omnibar orange in support user sessions

[arthur791004]

Proposed Changes

• Recolor the Multi-site Dashboard's interim omnibar (Calypso's masterbar) orange while a support user session is active, mirroring Calypso's support-session masterbar indicator.
• Adds an is-support-user-session class to the #wpcom-omnibar mount in client/dashboard/app/interim-omnibar/index.tsx when isSupportUserSession() is true.
• Overrides the masterbar color custom properties (--color-masterbar-background, --color-masterbar-border, --color-masterbar-item-hover-background, --color-masterbar-item-active-background) to Color Studio orange shades in interim-omnibar/style.scss. The dashboard already themes the masterbar entirely through these properties, so overriding them is sufficient — no new selectors fight the masterbar stylesheet.

Why are these changes being made?

Support agents need an unmistakable visual cue that they're operating inside a customer's session. Calypso's classic masterbar turns orange during support sessions; the dashboard's interim omnibar had no equivalent indicator. This brings parity.

The indicator is gated on support "user" sessions only (isSupportUserSession()), not "next" sessions — matching the existing components/header-bar decision, whose comment notes that "next" sessions already have a floating toolbar acting as their visual indicator.

The Color Studio custom properties aren't loaded globally in the dashboard, so each value uses a var(--studio-orange*, <hex fallback>) form (the same pattern already used in components/segmented-bar), keeping the real palette when present and a correct fallback otherwise.

Testing Instructions

• Start Calypso (yarn start) and open the Multi-site Dashboard (http://my.localhost:3000/sites).
• Enter a support user session (impersonate a user via the support tooling so sessionStorage.boot_support_user is set at boot).
• Confirm the top omnibar/masterbar renders orange instead of the default dark toolbar, and that item hover/active states use darker orange shades.
• Confirm a normal (non-support) session, and a support "next" session, are unaffected (dark toolbar as before).
• Quick CSS-only check without a real session: on the dashboard, run document.getElementById('wpcom-omnibar').classList.add('is-support-user-session') in the console and confirm the bar turns orange.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• For UI changes, have you tested the affected components in dark mode?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations?
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)? (No new strings.)
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

🤖 Generated with Claude Code

Screenshot

Masterbar during a support "user" session (orange, matching Calypso):

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-187011

Jetpack Cloud Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-187011&env=jetpack

Automattic for Agencies Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-187011&env=a8c-for-agencies

Dashboard Live (dotcom) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-187011&env=dashboard

Dashboard Live (A4A) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-187011&env=dashboard-a4a

[StevenDufresne]

LGTM. I think the hard coded variables are fine. Ideally we wouldn't need to ship them with the main app but they are minor.

[fushar]

Code changes look good.