Let double-click drill down from files, not just folders

[phalladar]

Summary

Drill-down only worked on folders. Files cover most of the treemap's area, so double-clicking almost always did nothing, and the right-click Drill Down item rarely appeared.

• Double-clicking a file now drills into the folder that contains it. Double-clicking a folder works as before.
• Nothing happens if the target folder is already the one shown, e.g. a file directly in the current folder.
• The right-click menu uses the same rule and names the target: Drill Down into <folder>.
• The README mentions drill-down again, as a how-to step and a feature, with the breadcrumb bar and Back button as the way back up.

Testing

• swift build passes.
• A build stamped as SDK 26.5 passes scripts/smoke-test.sh (from CI: launch the app before merging and before publishing a release #13).
• Manually tested in the app: double-clicking a file drills into its folder, breadcrumbs and Back return up, and the right-click "Drill Down into …" item works.

🤖 Generated with Claude Code