Improve snapshot browser single-child expansion

[jaysonsantos]

Summary

• Auto-expand single-child snapshot browser paths on the backend until the listing reaches a directory with multiple entries, a file, an empty directory, or the max expansion depth.
• Return the deepest expanded path from ListSnapshotFiles so the frontend can reconstruct and render the collapsed path chain with one API request per click.
• Add tests for snapshot path normalization and direct-child filtering.
• Ignore .direnv/ for local Nix/direnv development state.

Testing

• gzip -c /dev/null > webui/dist/index.html.gz && nix shell nixpkgs#go --command go test ./...
• nix shell nixpkgs#go --command go test -race ./internal/orchestrator/repo ./internal/api ./webui
• cd webui && npm run check
• cd webui && npm run build
• Manual browser test of the snapshot browser against a dev Backrest instance.

Disclosure

This change was implemented with the help of an AI coding agent. I manually reviewed the code and manually tested the feature before opening this PR.

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