fix(store): accept lowercase drive letters in root_path integrity check

The projects.root_path integrity check only accepted '/' or an uppercase
'A'-'Z' first character. On Windows, drive letters are commonly lowercase
(c:/repo, y:/share), so such a path was flagged store.corrupt and the DB
was auto-deleted on open — a likely cause of the mapped-drive DB deletion
in #227/#367. Accept 'a'-'z' as well.

Relates to #227, #367.