feat(i18n): Settings calendar 'requires readpst' string

src/lib/i18n/en.json

@@ -107,6 +107,7 @@
   "settings.calendar.title": "Calendar & Outlook .pst",
   "settings.calendar.intro_1": "Import events and attendees from a local Outlook",
   "settings.calendar.intro_2": "backup — read-only, nothing is written back to the file. Nothing leaves this device.",
+  "settings.calendar.requires": "Needs this on your PATH (not bundled):",
   "settings.calendar.pst_file": ".pst file",
   "settings.calendar.no_file": "No file selected",
   "settings.calendar.browse": "Browse…",