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.

po/pdfs.pot

@@ -8,7 +8,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: proton-drive-linux 3.6.0\n"
 "Report-Msgid-Bugs-To: https://github.com/narrrl/proton-drive-linux/issues\n"
-"POT-Creation-Date: 2026-10-26 22:28+0100\n"
+"POT-Creation-Date: 2026-10-31 10:37+0100\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"
@@ -397,14 +397,25 @@ msgstr ""
 msgid "The Proton Drive service isn't running."
 msgstr ""
 
-#: crates/pdfs-gui/src/app/main.rs:1803
+#: crates/pdfs-gui/src/app/main.rs:1813
+msgid "Can't open your Drive"
+msgstr ""
+
+#: crates/pdfs-gui/src/app/main.rs:1816
+#, rust-format
+msgid ""
+"The Proton Drive service keeps failing to start and tries again on its own. "
+"The last error was: {error}"
+msgstr ""
+
+#: crates/pdfs-gui/src/app/main.rs:1824
 #: crates/pdfs-gui/src/app/pages/status.rs:355
 #: crates/pdfs-gui/src/hud/mod.rs:269 crates/pdfs-gui/src/hud/mod.rs:579
-#: crates/pdfs-gui/src/main.rs:475 crates/pdfs-gui/src/main.rs:517
+#: crates/pdfs-gui/src/main.rs:493 crates/pdfs-gui/src/main.rs:535
 msgid "Connecting…"
 msgstr ""
 
-#: crates/pdfs-gui/src/app/main.rs:1804
+#: crates/pdfs-gui/src/app/main.rs:1825
 msgid "Waiting for the Proton Drive service to start."
 msgstr ""
 
@@ -2788,7 +2799,7 @@ msgid "Sync paused until you resume"
 msgstr ""
 
 #: crates/pdfs-gui/src/app/pages/locations.rs:570
-#: crates/pdfs-gui/src/app/pages/status.rs:1445 crates/pdfs-gui/src/main.rs:290
+#: crates/pdfs-gui/src/app/pages/status.rs:1445 crates/pdfs-gui/src/main.rs:308
 msgid "Sync paused"
 msgstr ""
 
@@ -4883,7 +4894,7 @@ msgstr ""
 msgid "Until you resume"
 msgstr ""
 
-#: crates/pdfs-gui/src/app/pages/status.rs:1464 crates/pdfs-gui/src/main.rs:292
+#: crates/pdfs-gui/src/app/pages/status.rs:1464 crates/pdfs-gui/src/main.rs:310
 #, rust-format
 msgid "{n} change needs attention"
 msgid_plural "{n} changes need attention"
@@ -6071,52 +6082,57 @@ msgstr ""
 msgid "Signed in as {username} — locked"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:200
+#: crates/pdfs-gui/src/main.rs:209
+#, rust-format
+msgid "Signed in as {username} — can't open your Drive"
+msgstr ""
+
+#: crates/pdfs-gui/src/main.rs:218
 #, rust-format
 msgid "Signed in as {username} — not connected"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:205
+#: crates/pdfs-gui/src/main.rs:223
 msgid "Not signed in"
 msgstr ""
 
 #. Translators: {error} is an error message, in English.
-#: crates/pdfs-gui/src/main.rs:208
+#: crates/pdfs-gui/src/main.rs:226
 #, rust-format
 msgid "Error: {error}"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:230
+#: crates/pdfs-gui/src/main.rs:248
 msgid "Proton Drive sent an unexpected reply"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:284
+#: crates/pdfs-gui/src/main.rs:302
 msgid "Session expired — sign in again"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:286
+#: crates/pdfs-gui/src/main.rs:304
 msgid "Sign-in not saved — unlock your keyring"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:289
+#: crates/pdfs-gui/src/main.rs:307
 msgid "Update not finished — restart Proton Drive"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:299
+#: crates/pdfs-gui/src/main.rs:317
 #, rust-format
 msgid "Up to date in {mountpoint} ({n} pinned)"
 msgid_plural "Up to date in {mountpoint} ({n} pinned)"
 msgstr[0] ""
 msgstr[1] ""
 
-#: crates/pdfs-gui/src/main.rs:306
+#: crates/pdfs-gui/src/main.rs:324
 #, rust-format
 msgid "Syncing — {queued} ({n} pinned)"
 msgid_plural "Syncing — {queued} ({n} pinned)"
 msgstr[0] ""
 msgstr[1] ""
 
-#: crates/pdfs-gui/src/main.rs:313
+#: crates/pdfs-gui/src/main.rs:331
 #, rust-format
 msgid "Offline — cached files only ({n} pinned)"
 msgid_plural "Offline — cached files only ({n} pinned)"
@@ -6124,60 +6140,60 @@ msgstr[0] ""
 msgstr[1] ""
 
 #. Translators: {queued} is a queue summary such as "3 uploads queued".
-#: crates/pdfs-gui/src/main.rs:319
+#: crates/pdfs-gui/src/main.rs:337
 #, rust-format
 msgid "Offline — {queued}"
 msgstr ""
 
 #. Translators: a menu item that opens the list of failing changes.
-#: crates/pdfs-gui/src/main.rs:358
+#: crates/pdfs-gui/src/main.rs:376
 #, rust-format
 msgid "{n} issue — View"
 msgid_plural "{n} issues — View"
 msgstr[0] ""
 msgstr[1] ""
 
-#: crates/pdfs-gui/src/main.rs:465
+#: crates/pdfs-gui/src/main.rs:483
 msgid "Sign In…"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:466
+#: crates/pdfs-gui/src/main.rs:484
 msgid "Sign In Again…"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:469
+#: crates/pdfs-gui/src/main.rs:487
 msgid "Open Proton Drive"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:473
+#: crates/pdfs-gui/src/main.rs:491
 msgid "Restart Proton Drive"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:482
+#: crates/pdfs-gui/src/main.rs:500
 msgid "Open Folder"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:494
+#: crates/pdfs-gui/src/main.rs:512
 msgid "Resume Sync"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:496
+#: crates/pdfs-gui/src/main.rs:514
 msgid "Pause Sync"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:515
+#: crates/pdfs-gui/src/main.rs:533
 msgid "Connect"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:522
+#: crates/pdfs-gui/src/main.rs:540
 msgid "Hide Tray Icon"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:526
+#: crates/pdfs-gui/src/main.rs:544
 msgid "Stop Proton Drive…"
 msgstr ""
 
-#: crates/pdfs-gui/src/main.rs:530
+#: crates/pdfs-gui/src/main.rs:548
 msgid "Quit"
 msgstr ""