Key the syncer, local store and IMAP server by Tuta MailSet folder id
instead of the system folder kind, so custom (and nested) folders are
first-class.
- tuta.rs: add FolderInfo; MailBackend.list_folders enumerates system +
custom folders via the SDK FolderSystem tree, building IMAP paths and
RFC 6154 special-use flags; load mails by the folder's entries list.
- store.rs: folder_kind INTEGER -> folder_id TEXT, schema v2 with a
migration that drops the cache tables and re-syncs (encrypted .eml
files survive).
- sync.rs: MailStore keyed by folder id; the syncer enumerates the live
folder list each cycle.
- imap/session.rs: dynamic LIST/SELECT/STATUS driven by the folder list;
drop the hardcoded six-folder mapping.
Pulls in the SDK FolderSystem tree (tuta-repo submodule bump).
Live-tested: 9 custom folders listed and selectable over IMAP, with
headers and decrypted bodies. Not yet covered: nested-folder paths,
modified UTF-7 for non-ASCII names, labels, and IMAP MOVE/COPY.
