fix(changes): compact single-child folders

[BenLocal]

Summary

Improve the Changes sidebar by displaying deep single-child directory chains in VS Code-style compact folders.

For example:

src
  main
    java
      App.java

is now displayed as:

src / main / java
  App.java

## Changes

- Compact consecutive directories when each directory has only one child directory.
- Stop compacting at directory branches or when a directory also contains files.
- Preserve the deepest real directory path for:
    - Expand and collapse
    - Folder selection
    - Patch creation
    - Stash restoration

- Truncate long folder labels while exposing the full path through the native tooltip.
- Use iterative chain folding to avoid stack overflow on pathological deep paths.
- Apply the same behavior to committed, staged, unstaged, compare, and stash views.

## Testing

Added regression coverage for:

- Single-child directory chains
- Directory branches
- Directories containing both files and subdirectories
- Staged and unstaged changes
- Folder expand/collapse
- Folder patch paths
- A pathological 3,000-level directory chain

Validation results:

- 126 test files passed
- 1,879 tests passed, 9 skipped
- Statement coverage: 98.01%
- Branch coverage: 90.39%
- TypeScript check passed
- Svelte check passed with 0 errors and 0 warnings
- Production build passed
- Dependency audit reported 0 vulnerabilities