fix(app): reuse open project root for nested folders

[tsubasakong]

Issue for this PR

Closes #16645

Type of change

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

What does this PR do?

Issue #16645 reports that opening a nested child folder from an already-open repo adds a duplicate project entry and can strand navigation on the nested path instead of the original repo.

This PR adds a small workspace-root helper that prefers the deepest already-open project root containing the selected directory. The project-open and project-navigation paths now reuse that canonical root, so selecting a nested folder inside an already-open repo reopens the existing project instead of creating a second project entry.

How did you verify your code works?

I verified the touched path-resolution logic locally by reviewing the new helper coverage and running git diff --check to confirm the patch is clean.

I also attempted to run bun test --preload ./happydom.ts ./src/pages/layout/helpers.test.ts from packages/app, but this checkout does not currently have the app dev dependency @happy-dom/global-registrator installed, so the unit test could not run in this environment.

Screenshots / recordings

Not included for this focused path-resolution fix.

Checklist

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

[natewill]

This is similar to #16684, but only fixes #16645 and not #16713