refactor: item 29 cleanup

F16 F17 F18 F19 F20 F21: import polling backs off and pauses in a hidden tab, one date format module, fake timers in the import tests, error codes instead of raw messages, one safe storage wrapper, dead code.

public/i18n/de.json

@@ -542,7 +542,7 @@
     "BACK_TO_SOURCE": "Zurück zur Quelle",
     "CORRECTED": "Korrigiert",
     "SAVED": "Korrektur gespeichert",
-    "INVALID": "Nicht gespeichert: {{message}}",
+    "INVALID": "Nicht gespeichert: Der Wert passt nicht zu diesem Feld.",
     "NO_ACCESS": "Nur Auditoren können Spieldaten korrigieren.",
     "ERROR": "Die Korrektur konnte nicht gespeichert werden.",
     "IMAGE_HINT": "https://… (leer: das Steam-Headerbild)",

public/i18n/en.json

@@ -542,7 +542,7 @@
     "BACK_TO_SOURCE": "Back to source",
     "CORRECTED": "Corrected",
     "SAVED": "Correction saved",
-    "INVALID": "Not saved: {{message}}",
+    "INVALID": "Not saved: the value does not fit this field.",
     "NO_ACCESS": "Only auditors can correct game data.",
     "ERROR": "The correction could not be saved.",
     "IMAGE_HINT": "https://… (empty: the Steam header)",