Storage default `D:/Gently3` is non-portable — creates a stray relative `D:` dir off-Windows

[subindevs]

Problem

StorageSettings.base_path defaults to a Windows drive path:

# gently/settings.py:73
base_path: Path = field(default_factory=lambda: _env("STORAGE_PATH", Path("D:/Gently3")))

On Windows (the microscope PCs) this is correct — D: is the dedicated data drive. But off-Windows (Linux dev boxes, macOS, CI), Path("D:/Gently3") is not an absolute path. It resolves relative to the cwd, so a default-config run silently creates a junk directory literally named D: and writes session data into it.

PR #51 added D:/ to .gitignore, which hides the symptom but doesn't stop the app from writing to a nonsense location.

Actual fix

Make the default platform-aware so Windows is untouched and everywhere else gets a real absolute path:

def _default_storage_path() -> Path:
    if os.name == "nt":
        return Path("D:/Gently3")
    xdg = os.environ.get("XDG_DATA_HOME")
    base = Path(xdg) if xdg else Path.home() / ".local" / "share"
    return base / "Gently3"

# ...
base_path: Path = field(default_factory=lambda: _env("STORAGE_PATH", _default_storage_path()))

• Windows production behavior is identical.
• GENTLY_STORAGE_PATH override is unchanged.
• The .gitignore D:/ entry becomes unnecessary (can be left as a stopgap).

Out of scope / follow-up

Other hardcoded D:/... defaults exist in legacy/script modules and could later route through settings.storage.base_path:

• gently/dataset/aggregator.py, embryo_dataset.py:164, schema.py:24 (DEFAULT_DB_PATH)
• benchmarks/perception/runner.py:493, diagnostics/segment_embryo_nuclei.py:57

Notes

Behavior change on non-Windows defaults → land as its own PR against development, not folded into release #51.

[pskeshu]

This is the way 😁

[pskeshu]

Note for whoever picks this up: CLAUDE.md needs updating as part of the fix.

Its "Running the app off-Windows" section currently documents the present
behaviour — that the default is not an absolute path off-Windows, that it
silently creates a junk D: directory, and that GENTLY_STORAGE_PATH must
therefore be set explicitly on Linux/macOS.

Once the platform-aware default lands, that guidance becomes wrong: the
explicit-path instruction stops being necessary, and the warning describes
behaviour that no longer exists. Please update or drop that section in the same
PR so the doc doesn't outlive the bug.