fix(app): list folders in empty project picker

[ndj888]

Issue for this PR

Closes #37611

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Treats an empty directory-picker filter as directory browsing instead of fuzzy search. The initial Add Project dialog now calls file.list() for its base directory and displays child folders immediately.

Non-empty name searches still use find.files().

How did you verify your code works?

• Reproduced with opencode web --hostname 0.0.0.0 --port 8088: the picker initially requested find/file with an empty query and displayed no folders.
• Ran the patched app against that server in an isolated Chrome context.
• Verified Add Project immediately listed the home-directory folders with no input.
• Selected ~/IdeaProjects; the dialog closed and the project appeared in the sidebar.
• Ran the directory-picker unit tests (20 passed), app typecheck, and Prettier checks.

Screenshots / recordings

Verified in the Web UI that the initial picker lists folders before any search text is entered.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR