feat(plex): Settings toggle for Plex watch-sync + one-click import

Admins get a 'Plex watch sync' section in Settings → Account (shown when the Plex module is on):
a two-way-sync toggle whose first enable imports the Plex watch history, a last-import line, and an
'Import from Plex now' button. api.plexWatch{Link,SetLink,Import}; trilingual EN/HU/DE strings.

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

@@ -5,6 +5,16 @@
     "notifications": "Notifications",
     "account": "Account"
   },
+  "plexSync": {
+    "title": "Plex watch sync",
+    "intro": "Sync your watched / in-progress status between Plex and Siftlode. Turning this on imports your existing Plex history now (Plex wins); ongoing two-way sync follows.",
+    "toggle": "Two-way sync with Plex",
+    "toggleHint": "Uses this server's Plex account. Turning it on imports your Plex watch history once.",
+    "importNow": "Import from Plex now",
+    "imported": "Imported from Plex: {{watched}} watched, {{in_progress}} in progress.",
+    "lastSync": "Last import: {{when}}",
+    "never": "Not imported yet"
+  },
   "save": {
     "unsaved": "Unsaved changes",
     "saving": "Saving…",