i18n(de): complete German translation (4 missing strings, 2 fuzzy fixes)

- Translate the 4 remaining empty entries (reverse-proxy hints, Apply, Go to page)
- Fix 2 fuzzy entries in helper.py conversion messages: the old msgstr was a
  copy of 'format not found' and referenced %(format)s, which is not a
  placeholder of these msgids (would raise KeyError at runtime)
- German is now 100% (0 untranslated, 0 fuzzy), file compiles with msgfmt/polib

Author: urs-eller

cps/translations/de/LC_MESSAGES/messages.po

@@ -1379,14 +1379,14 @@ msgid "This Email has been sent via Calibre-Web Automated."
 msgstr "Diese E-Mail wurde von Calibre-Web Automated gesendet."
 
 #: cps/helper.py:129
-#, fuzzy, python-format
+#, python-format
 msgid "Conversion timed out for book id: %(book)d"
-msgstr "%(format)s Format für Buch-ID %(book)d nicht gefunden"
+msgstr "Zeitüberschreitung bei der Konvertierung für Buch-ID: %(book)d"
 
 #: cps/helper.py:131
-#, fuzzy, python-format
+#, python-format
 msgid "Conversion failed for book id: %(book)d"
-msgstr "%(format)s Format für Buch-ID %(book)d nicht gefunden"
+msgstr "Konvertierung fehlgeschlagen für Buch-ID: %(book)d"
 
 #: cps/helper.py:139
 msgid "Calibre-Web Automated Test Email"
@@ -2849,7 +2849,7 @@ msgstr "Externer Port"
 
 #: cps/templates/admin.html:123
 msgid "Not set — reverse proxy detected"
-msgstr ""
+msgstr "Nicht gesetzt — Reverse Proxy erkannt"
 
 #: cps/templates/admin.html:134 cps/templates/config_view_edit.html:45
 msgid "Books per Page"
@@ -3287,7 +3287,7 @@ msgstr "Lade..."
 
 #: cps/templates/book_edit.html:312
 msgid "Apply"
-msgstr ""
+msgstr "Übernehmen"
 
 #: cps/templates/book_edit.html:369
 msgid "Search error!"
@@ -3765,7 +3765,7 @@ msgstr "Externer Server-Port (für Portweiterleitung von API-Aufrufen)"
 msgid ""
 "A reverse proxy was detected. Leave blank unless you need to override the "
 "port for direct access."
-msgstr ""
+msgstr "Ein Reverse Proxy wurde erkannt. Leer lassen, außer der Port für den Direktzugriff soll überschrieben werden."
 
 #: cps/templates/config_edit.html:217
 msgid "Sync Kobo read progress to Hardcover (Requires API key per user)"
@@ -6419,7 +6419,7 @@ msgstr "Vorheriger Eintrag"
 
 #: cps/templates/layout.html:353
 msgid "Go to page"
-msgstr ""
+msgstr "Gehe zu Seite"
 
 #: cps/templates/layout.html:376
 msgid "Book Details"