fix(fuse,core,cli,gui): show a refused session as expired, not offline

A refresh token Proton refused, or a deleted or disabled account, was
treated as a lost connection: the mount came up "offline", the probe
retried forever and logged only at debug, and nothing told the user that
only signing in again helps (issue #35).

session_revoked() names those errors. The root fetch, probe, drain,
event poll, event seed and photos root now set session_expired, log one
warning naming `pdfs login`, and stop retrying. Cached files stay
readable and changes stay queued.

Response::Status carries session_expired (serde default false). The
tray shows "Session expired — sign in again" with a Sign In Again…
item that opens the app's sign-in page; the app's status and Sync card
say the same with a button. `pdfs status` and `pdfs diagnose` report it.
New strings are translated in every catalog.

docs/BUGS.md B202.

crates/pdfs-core/src/control.rs

@@ -1959,6 +1959,11 @@ pub enum Response {
         /// resumed, or not paused.
         #[serde(default)]
         paused_until: Option<i64>,
+        /// True when Proton refused the session itself (refresh token refused,
+        /// account deleted or disabled). `online` is false then too, but no
+        /// connection brings it back: only signing in again does.
+        #[serde(default)]
+        session_expired: bool,
     },
     /// A human-readable success message.
     Ok { message: String },