fix: consolidate data file locations to follow XDG Base Directory Specification

[swarnimbandekar]

Moves data files from $HOME to proper XDG directories:
fixes #76

Before	After
~/.pyvm_history.json	~/.local/share/pyvm/history.json
~/.pyvm_metadata.sqlite	~/.cache/pyvm/metadata.sqlite
~/.pyvm/venvs/	~/.local/share/pyvm/venvs/
~/.pyvm/venvs.json	~/.local/share/pyvm/venvs.json

On Windows everything goes to %LOCALAPPDATA%/pyvm/.

Existing files are automatically migrated on first run. Migration only runs once and logs what it moved.

Also respects XDG_*_HOME env vars if set.

Testing: All 64 tests pass, including 15 new ones for path resolution and migration.

GSSoC26

[shreyasmene06]

@swarnimbandekar please make sure the checks are passed

[shreyasmene06]

The XDG migration logic can lose user data in conflict cases:

1. _move_file: if destination already exists, it deletes the legacy source (src.unlink()), which can discard data.
2. _move_directory: when destination exists, conflicting items are skipped, then source dir is removed (rmtree), so
   skipped/conflicting files can be lost.

Please make migration conflict-safe (no destructive delete of source on conflict), and add tests for “destination already
exists / partial overlap” cases before merge.

[swarnimbandekar]

fix:
Resolved silent data loss issues during migration to XDG-compliant locations by ensuring that when destination files or directories already exist, legacy data is preserved in place (not deleted), and only non-conflicting items are moved. Added safety checks so migration does not mark as complete if conflicts occur, enabling retries. Relevant tests have been added and all pass.

[shreyasmene06]

Thanks for the fixes. The previous conflict/data-loss issue looks addressed and CI is green, but I still see one blocker
before merge.

When migrating ~/.pyvm/venvs and ~/.pyvm/venvs.json, the registry file is moved as-is. Existing registry entries can
still contain absolute paths under the legacy root, e.g. ~/.pyvm/venvs/myenv, while the actual venv directory has been
moved to the new XDG location, e.g. ~/.local/share/pyvm/venvs/myenv.

That would make migrated venvs appear missing or fail activation/removal because venv.py trusts the stored registry path.

Please update the migration to rewrite registry entry paths from the old venv root to the new venv root, and add a test
covering an existing venvs.json entry with an absolute legacy path.

Also minor: README.md line 251 has trailing whitespace (git diff --check catches it).

[shreyasmene06]

LGTM!
Thank you for your contribution