feat(app): add sidebar sub-project grouping

[anduimagui]

Supersedes #16229 (branch renamed).

Issue for this PR

Closes #

Type of change

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

What does this PR do?

This adds sub-project grouping in the sidebar so projects can be organized under a parent project and managed from the project context menu.

Key changes:

• Added "Add sub-project" and "Remove from sub-project group" actions in the sidebar project menu.
• Added parent/child project state management and grouped rendering behavior in layout state.
• Ensured navigation expands the relevant parent group when entering a sub-project.
• Added helper utilities and tests for project grouping behavior and stable group IDs/labels.
• Added picker defaultPath support so sub-project selection starts from the parent project path (including WSL path handling).
• Added i18n strings and a small sidebar sub-project entry animation.

How did you verify your code works?

• Added/updated unit tests in packages/app/src/pages/layout/helpers.test.ts for:
  • stable group id generation
  • group label formatting
  • all-project and parent-group creation
  • sub-project grouping under selected core project
• Manually reviewed integration points in:
  • packages/app/src/pages/layout.tsx
  • packages/app/src/pages/layout/sidebar-project.tsx
  • packages/app/src/pages/layout/sidebar-shell.tsx
  • packages/desktop/src/index.tsx

Screenshots / recordings

UI change: please attach before merge.

• Sidebar context menu showing "Add sub-project" / "Remove from sub-project group"
• Sidebar with expanded parent project and visible sub-projects

Checklist

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

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