feat(i18n): translate login and app chrome (HU/EN/DE)

Login screen (with a language picker), header, account menu, sync status, About and
Release Notes dialogs, and the version banner are now fully translated in Hungarian,
English and German.

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

@@ -0,0 +1,32 @@
+{
+  "feed": "Feed",
+  "searchPlaceholder": "Search your subscriptions…",
+  "channelManager": "Channel manager",
+  "usageStats": "Usage & stats",
+  "account": {
+    "admin": "Admin",
+    "feed": "Feed",
+    "channels": "Channels",
+    "stats": "Stats",
+    "settings": "Settings",
+    "about": "About",
+    "signOut": "Sign out"
+  },
+  "sync": {
+    "yours": "yours",
+    "total": "total",
+    "countTooltip": "Videos from your subscriptions, against the total in the shared catalog that every user's subscriptions have pulled in.",
+    "paused": "paused",
+    "syncing": "{{count}} syncing",
+    "allSynced": "all synced",
+    "withoutFullHistory": "{{count}} without full history",
+    "fullHistoryTooltip": "Some of your channels only have their recent uploads. Click to open the channel manager (filtered to these) and request their full back-catalog.",
+    "pause": "Pause background sync",
+    "resume": "Resume background sync"
+  },
+  "banner": {
+    "updated": "Updated to v{{version}} — see what's changed.",
+    "releaseNotes": "Release notes",
+    "dismiss": "Dismiss"
+  }
+}