Follow-up hardening to #789. The selection was keyed purely by folder name,
so renaming a music folder in Navidrome silently reverted the scan to all
libraries. Now persist the folder id (stable across renames) as the primary
key alongside the name (kept for display + back-compat), and restore by id
first with a name fallback. Self-heals on reconnect: pre-id installs and
drifted/renamed names get the id + fresh name written back, so the settings
dropdown keeps highlighting the right folder.
Tests: restore-by-id-after-rename (+ name heal), name-fallback self-heals id,
no-drift writes nothing.
85b6ddb997