Shell-expand pimdir.root

A pimdir account pointed at a Neverest store with the natural pimdir.root = "~/.local/state/neverest/posteo" had an empty mailbox list (and every read empty). pimdir.root is a PathBuf deserialized verbatim with no ~ expansion, and PimdirStore::open creates on absence — so himalaya opened the literal relative path ./~/.local/…, made an empty store there, and listed nothing, with no error (and littered a ~ dir in the cwd).

Fix: PimdirClient::new expands ~/env vars on root with shellexpand::full (already a dep, used by the wizard) before opening the store and blobs, falling back to the raw path on failure. Also: config.sample.toml now documents pimdir.root (one line to read a Neverest account) and clarifies that pimdir.source is auto-detected and normally omitted; the PimdirConfig.source doc was corrected likewise.

Verified read-only against the real Neverest-synced Posteo store: mailbox list returns all 16 synced mailboxes; envelope list -m Notes renders subjects/senders from the v:1 meta; message read <id> pulls the body from the content-addressed blob and renders its MIME parts. Build + fmt clean.

Spec updated: backends (ADDED "pimdir store path is shell-expanded").