fix(app): browse subfolders in the open project dialog - #51533

Issue for this PR

Closes #42784

Type of change

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

What does this PR do?

The web "Add project" dialog lists only the direct children of the home directory. Subfolders can only be reached through the Tab autocomplete, which isn't discoverable.

This adds pointer navigation:

• a chevron on each folder row enters that folder (clicking the row still selects it)
• an up button next to the search box returns to the parent, and is disabled at the root

The current-directory logic is a pure helper (pickerBrowseDirectory) with unit tests, so the up button stays disabled for plain searches, and paths like ~, /, C:/, and UNC shares are handled. Builds on the auto-closed #42786.

How did you verify your code works?

• bun typecheck in packages/app
• bun test in packages/app (725 tests pass, including new directory-picker cases)
• Manually clicked through the Add project dialog in the web app

Screenshots / recordings

Before (no chevrons — row click selects immediately):

After (chevron + up button; chevron darkens on hover):

Checklist

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