feat: add immutable folder workspace binding

[4pmtong]

Related Issue

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Description

Adds desktop folder workspace selection with immutable project-level binding.

Users can now select a local folder for a project in desktop mode. Once selected, that folder becomes the project workspace and cannot be changed for the same project. New projects can still choose a different folder or start from scratch.

Key changes:

• Added Brain workspace APIs for capabilities, folder binding, and workspace state.
• Added immutable local workspace binding stored by sanitized email + project id.
• Added task directory snapshots so historical tasks resolve to the correct output root.
• Updated agent execution working directory resolution:
  • bound projects run directly in the selected folder
  • unbound projects keep the legacy task directory behavior
• Added async Brain-side artifact handling for user uploads, workspace outputs, camel_logs, sync_steps, and artifact manifests.
• Updated /files listing, preview, and stream paths to resolve through project/task workspace context.
• Added desktop-only folder picker through Electron, while website mode keeps the existing flow.
• Updated project picker UI to show the selected folder name and route users to create a new project when the current one is already bound.
• Added tests for workspace binding, path resolution, artifact handling, and environment working directory behavior.

Testing Evidence (REQUIRED)

• I have included human-verified testing evidence in this PR.
• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).
• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline