feat: Add local webui

[ncrispino]

To run, use uv run massgen --webui. You can then select the config to use in the top right of the UI. Answers will stream and notifications will be shown. Workspaces can also be browsed during execution. After votes are done, the final agent is selected then it moves to a dedicated screen with only that final agent to stream the final answer. At the end, the answer and workspace browser is shown in a way that enables easy copying.

Should work on simple tasks, though haven't tried longer horizon tasks with a lot of code writing or code only execution mode with docker.

May also need to cd webui; npm run build to get backend changes.

Closes MAS-122

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