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/about.json

@@ -0,0 +1,12 @@
+{
+  "title": "About",
+  "tagline": "Self-hosted, multi-user reader for your own YouTube subscriptions.",
+  "frontend": "Frontend",
+  "backend": "Backend",
+  "database": "Database",
+  "buildDate": "Build date",
+  "whatsNew": "What's new",
+  "releaseNotes": "Release notes",
+  "new": "New",
+  "fixes": "Fixes"
+}

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

@@ -0,0 +1,12 @@
+{
+  "privacyPolicy": "Privacy Policy",
+  "termsOfService": "Terms of Service",
+  "close": "Close",
+  "cancel": "Cancel",
+  "save": "Save",
+  "loading": "Loading…",
+  "language": "Language",
+  "somethingWrong": "Something went wrong.",
+  "accessRequestsTitle": "Access requests",
+  "accessRequestsMessage": "{{count}} pending — review in Settings → Account."
+}

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"
+  }
+}

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

@@ -0,0 +1,12 @@
+{
+  "tagline": "A self-hosted, filterable feed of your YouTube subscriptions — sort, tag, and tune out the noise. Sign in with Google to get started; YouTube access is an optional, separate step you control.",
+  "signIn": "Sign in with Google",
+  "approved": "You're already approved — just sign in with Google above.",
+  "requested": "Thanks — your request is in. We'll email you when it's approved.",
+  "noAccessYet": "No access yet?",
+  "denied": "That Google account isn't approved. Request access below.",
+  "emailPlaceholder": "you@gmail.com",
+  "sending": "Sending…",
+  "requestAccess": "Request access",
+  "submitError": "Couldn't submit — check the address and try again."
+}