fix(plex): optimistic toggle + 'importing' indicator for watch-sync

The watch-sync switch only flipped once the (multi-second) import request returned, so a click read as
'nothing happened' and invited repeat clicks. Reflect the toggle's target optimistically while the
mutation is in flight and show an 'Importing your Plex watch history…' spinner line, so it's obvious
the switch responded and is working.

frontend/src/i18n/locales/de/settings.json

@@ -13,7 +13,8 @@
     "importNow": "Jetzt aus Plex importieren",
     "imported": "Aus Plex importiert: {{watched}} gesehen, {{in_progress}} laufend.",
     "lastSync": "Letzter Import: {{when}}",
-    "never": "Noch nicht importiert"
+    "never": "Noch nicht importiert",
+    "importing": "Plex-Wiedergabeverlauf wird importiert…"
   },
   "save": {
     "unsaved": "Nicht gespeicherte Änderungen",

frontend/src/i18n/locales/en/settings.json

@@ -13,7 +13,8 @@
     "importNow": "Import from Plex now",
     "imported": "Imported from Plex: {{watched}} watched, {{in_progress}} in progress.",
     "lastSync": "Last import: {{when}}",
-    "never": "Not imported yet"
+    "never": "Not imported yet",
+    "importing": "Importing your Plex watch history…"
   },
   "save": {
     "unsaved": "Unsaved changes",

frontend/src/i18n/locales/hu/settings.json

@@ -13,7 +13,8 @@
     "importNow": "Importálás a Plexből most",
     "imported": "Importálva a Plexből: {{watched}} megnézve, {{in_progress}} folyamatban.",
     "lastSync": "Utolsó import: {{when}}",
-    "never": "Még nincs importálva"
+    "never": "Még nincs importálva",
+    "importing": "Plex-előzmény importálása…"
   },
   "save": {
     "unsaved": "Nem mentett változások",