feat: spaces, folder import, boards and mail; rename to narl-workspace

The diary grows into a workspace: a flat list of dated entries becomes a
tree of spaces, and the app is renamed to match what it now is.

Spaces and nodes
  One recursive `nodes` table replaces `entries`: a space is a root node,
  and folders and documents nest under it without limit. Entries keep
  their ids through the migration, so share tokens, media links and the
  drafts in localStorage all still resolve.

Folder import
  A folder or a zip is uploaded as a document tree. Markdown becomes
  documents titled from their first heading, everything else becomes
  media, and relative links between the imported files are rewritten to
  the paths they landed at, so a set of cross-linked notes stays
  navigable after the upload.

Boards
  A kanban board per space, with the space as the board: lists, cards,
  due dates and an optional link to a document. `diary` has none.

Mail
  SMTP notifications through an outbox. A mail is enqueued under a key
  naming the occurrence it belongs to, so a restart cannot resend and a
  relay that refuses is retried with a widening backoff. Four kinds: the
  diary reminder, cards due and overdue, a weekly digest, and logins
  from a browser that has not been seen before or a burst of failures.
  All of it is dormant until WORKSPACE_SMTP_URL is set.

Rename
  Everything user-visible says workspace; everything that already exists
  outside this repo keeps its name, because renaming it would break a
  running deployment for tidiness alone: the database file, the docker
  volume, the session cookie, the Proton device, the keyring entry and
  the localStorage keys. Configuration reads WORKSPACE_* first and falls
  back to DIARY_*, so the deployed .env keeps working untouched.

src/state.rs

@@ -2,7 +2,7 @@ use std::sync::Arc;
 
 use sqlx::SqlitePool;
 
-use crate::{backup::Backup, config::Config, throttle::LoginThrottle};
+use crate::{backup::Backup, config::Config, notify::Notifier, throttle::LoginThrottle};
 
 #[derive(Clone)]
 pub struct AppState {
@@ -11,4 +11,6 @@ pub struct AppState {
     pub login_throttle: Arc<LoginThrottle>,
     /// The Proton Drive mirror. Always present; dormant until it has a session.
     pub backup: Arc<Backup>,
+    /// Notification mail. Always present; dormant until WORKSPACE_SMTP_URL is set.
+    pub notify: Arc<Notifier>,
 }