Fix: Masterbar Horizon crash from string icon in createInterpolateElement

[p-jackson]

Proposed Changes

• In MasterbarLoggedIn, wrap the result of wordpressIcon() in a Gridicon when it returns a string, mirroring the approach used in client/layout/masterbar/item.tsx (line 102).

Rather than a broken button label, we now have this. It doesn't look great. But it wasn't working at all before.

Why are these changes being made?

• On Horizon, wordpressIcon() returns the string 'my-sites-horizon' instead of an SVG element. Since Masterbar: migrate from localize HOC to @wordpress/i18n #109489 migrated this code from i18n-calypso's translate() (with components) to @wordpress/element's createInterpolateElement, that string crashes the render: createInterpolateElement requires every value in the conversion map to be a React element, while the old translate() was forgiving and accepted strings. The bug was arguably latent before — the code only ever happened to pass elements — but the stricter API surfaced it on Horizon.
• The same string-or-element pattern exists in item.tsx, which already handles it with typeof icon !== 'string' ? icon : <Gridicon icon={ icon } size={ 24 } />. This PR uses the same shape for consistency.

Testing Instructions

• Load the masterbar against Horizon (hostname === 'horizon.wordpress.com') and confirm it renders without crashing and the WordPress.com account icon shows.
• Load on a non-Horizon environment and confirm the SVG icon still renders.

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?
• 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)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)?
• 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)?

[github-actions]

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[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• help-center
• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug fix/masterbar-wordpress-icon-horizon-crash on your sandbox.