[AI] Display beacon principal in shellv2 top bar

[KCarretto]

This PR updates the Shell v2 interface to make the beacon principal (the user identity the agent is running as) visible in the top bar. This improves visibility of the execution context without needing to hover over the beacon name.

Key changes:

• ShellHeader.tsx now conditionally renders a Badge containing the principal next to the beacon/host information.
• A new test file ShellHeader.test.tsx was added to ensure the header renders correctly with and without a principal.

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PR created automatically by Jules for task 16737458075365606044 started by @KCarretto

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