Add base path support for the observability dashboard

[akarshghale]

Please add support for hosting the observability dashboard under a configurable base path, for example:

/agent-browser

Current behavior:

• the dashboard appears to assume it is being served from /
• when it is mounted under a subpath, the initial UI shell may load, but the full dashboard does not behave correctly
• in practice, serving it behind a reverse proxy on a path prefix leads to incomplete or broken runtime behavior

What I need is for the dashboard to work correctly when it is not served from /, including:

• static assets resolving under the configured base path
• websocket/API/session/runtime requests using the same base path
• redirects and internal links preserving the base path
• overall dashboard behavior remaining the same as when hosted at root
• A configurable base path would make the dashboard much easier to integrate into existing applications and reverse-proxy setups where the dashboard needs to live under a subpath instead of at the origin root.

It would be helpful if this support included either:

• a runtime/config option for the dashboard base path, or
• documented guidance for how the dashboard is expected to behave behind a reverse proxy on a subpath

[akarshghale]

The dashboard frontend should not hardcode http://localhost:4848. It should derive its API and WebSocket endpoints from same-origin by default, with optional configurable public origin and base-path support for reverse-proxy deployments.