Fix local source path canonicalization for relative roots

[rmanibus]

Context

Backing up a local source specified as a relative path like local:./ can persist a broken display path in snapshot source metadata, for example:

/../../../Users/loichermann/workspace/cloudstic-cli

This happens when local source info tries to relativize the source path against the detected volume mount point even when the resolved paths do not actually share that root.

Goal

Persist a correct canonical local source path for relative roots, while still preserving portable volume-relative paths when the source really is inside the detected mount point.

Acceptance Criteria

• local:./ persists as a correct canonical absolute path when no safe volume-relative path can be derived
• volume-relative local paths still work when the source is genuinely inside the detected mount point
• tests cover the broken relative-root case and the root-containment helper logic