Enable creating projects pointing to existing directories (ENG-384, supersedes #210)

[tino097]

Description

Rebased continuation of #210 — original work by @MinuraPunchihewa (authorship preserved in the commits), rebased onto current staging. #210 stays open for its review history and demo recording; please review here.

Enables projects to be created by pointing at an existing local folder: a native directory picker in Electron (via a new shell:pick-directory IPC channel with a minimal preload surface), an explicit unsupported fallback path on web, and project instructions edited from the project page (stored on the Project record server-side instead of .anton/anton.md).

Changes on top of the original #210 branch:

• Rebase conflict resolutions keep staging's /projects/ endpoint paths, the session import in the Electron main process, and the ContextCard drag-and-drop overlay.
• The delete-project confirmation now shows the project's real folder path — deleting a project removes its folder from disk, which matters once that folder is one the user chose.

Fixes ENG-384

Back-end PR (merge first): mindsdb/cowork-server#155
Demo (from #210): https://www.loom.com/share/f3c82648de4b49e0bddb6b9768ab74ec

Type of change

• ⚡ New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have checked my code and corrected any misspellings

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