fix: only auto-open browser for rerun-web viewer backend

[Nabla7]

as the title suggests.

[greptile-apps]

Greptile Overview

Greptile Summary

Restricts automatic browser opening to only the rerun-web viewer backend. Previously, the browser would auto-open for all viewer backends (rerun-web, rerun-native, foxglove), which was inappropriate since rerun-native opens a separate native window and foxglove uses Foxglove Studio.

Key changes:

• Added conditional check: if self._global_config.viewer_backend == "rerun-web"
• Moved URL construction and logging inside the conditional block
• Updated comment to clarify the behavior for each viewer backend

This aligns with the documented viewer backend behavior where only rerun-web provides a full browser-based dashboard at the localhost URL.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change is a simple, well-scoped conditional that adds proper backend-specific behavior without modifying any existing logic for the rerun-web case. The fix addresses a user experience issue where browsers would inappropriately open for non-web viewer backends.
• No files require special attention

Sequence Diagram

sequenceDiagram
    participant User
    participant WebsocketVisModule
    participant GlobalConfig
    participant Browser
    participant UvicornServer

    User->>WebsocketVisModule: start()
    WebsocketVisModule->>WebsocketVisModule: _create_server()
    WebsocketVisModule->>WebsocketVisModule: _start_broadcast_loop()
    WebsocketVisModule->>UvicornServer: Start uvicorn server thread
    
    WebsocketVisModule->>GlobalConfig: Check viewer_backend
    
    alt viewer_backend == "rerun-web"
        GlobalConfig-->>WebsocketVisModule: "rerun-web"
        WebsocketVisModule->>WebsocketVisModule: Construct URL
        WebsocketVisModule->>WebsocketVisModule: Log Command Center URL
        WebsocketVisModule->>Browser: webbrowser.open_new_tab(url)
        Browser-->>User: Opens dashboard in browser
    else viewer_backend == "rerun-native" or "foxglove"
        GlobalConfig-->>WebsocketVisModule: "rerun-native" or "foxglove"
        Note over WebsocketVisModule,Browser: Browser NOT opened<br/>User accesses manually if needed
    end
    
    WebsocketVisModule->>WebsocketVisModule: Subscribe to data streams

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