fix(cli,core,fuse,gui): say why the mount keeps failing, and back off

A mount that failed on every try retried every 5 s forever, and the
reason was only in the journal: the app and the tray said Connecting...
for days (B263, issue #40). The retry now doubles to a 2-minute ceiling,
and the last failure is kept in mount-failure.json for the app, the
tray, pdfs diagnose and systemctl status.

The early returns in mount left the workers they had started running
beside the next attempt (B264). They are now stopped on every error
path, without telling systemd the daemon is stopping.

crates/pdfs-gui/src/app/main.rs

@@ -1797,13 +1797,35 @@ pub(crate) fn service_unreachable(
             );
             return;
         }
-        status(
-            &ui,
-            "network-idle-symbolic",
-            &gettext("Connecting…"),
-            &gettext("Waiting for the Proton Drive service to start."),
-            false,
-        );
+        // Running but not answering may be a mount that fails on every try,
+        // which never opens the socket. Its reason is in a file the daemon
+        // keeps for this (B263); "Connecting…" alone looked like a slow start
+        // for as long as it lasted.
+        let state_dir = ui.dirs.state_dir();
+        let failure = gio::spawn_blocking(move || pdfs_core::mountfail::read(&state_dir))
+            .await
+            .ok()
+            .flatten();
+        match failure {
+            Some(failure) => status(
+                &ui,
+                "dialog-warning-symbolic",
+                &gettext("Can't open your Drive"),
+                // Translators: {error} is an error message, in English.
+                &gettext_f(
+                    "The Proton Drive service keeps failing to start and tries again on its own. The last error was: {error}",
+                    &[("error", &failure.error)],
+                ),
+                true,
+            ),
+            None => status(
+                &ui,
+                "network-idle-symbolic",
+                &gettext("Connecting…"),
+                &gettext("Waiting for the Proton Drive service to start."),
+                false,
+            ),
+        }
         glib::timeout_add_local_once(CONNECT_RETRY_INTERVAL, move || {
             if ui.stack.visible_child_name().as_deref() == Some(page) {
                 reload(&ui);

crates/pdfs-gui/src/main.rs

@@ -194,6 +194,24 @@ fn poll_state(socket: &Path, default_mountpoint: &Path) -> DriveState {
                     ),
                     Phase::Attention,
                 ),
+                // A daemon whose mount fails on every try never opens the
+                // socket; the app says why (B263).
+                Ok(s)
+                    if socket
+                        .parent()
+                        .and_then(pdfs_core::mountfail::read)
+                        .is_some() =>
+                {
+                    (
+                        // Translators: {username} is the Proton account name. The
+                        // service keeps failing to start; the app shows the error.
+                        gettext_f(
+                            "Signed in as {username} — can't open your Drive",
+                            &[("username", &s.username)],
+                        ),
+                        Phase::Attention,
+                    )
+                }
                 Ok(s) => (
                     // Translators: {username} is the Proton account name.
                     gettext_f(