feat(config): admin Configuration page (registry-driven)

New admin-only Configuration page that renders the system_config registry grouped
(Email/SMTP first), with per-field save/reset, write-only encrypted secret fields
(disabled with a hint when TOKEN_ENCRYPTION_KEY is unset), and a Send-test-email
button. New 'config' page + sidebar nav item + header title; api methods and
EN/HU/DE strings.

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

@@ -0,0 +1,30 @@
+{
+  "intro": "Operational settings stored in the database — these override the file/env defaults and take effect without a redeploy. Leave a field empty (or reset it) to fall back to the default.",
+  "loading": "Loading configuration…",
+  "groups": {
+    "email": "Email / SMTP"
+  },
+  "fields": {
+    "smtp_host": { "label": "SMTP host", "hint": "e.g. smtp.gmail.com" },
+    "smtp_port": { "label": "SMTP port", "hint": "Usually 587 (STARTTLS)." },
+    "smtp_user": { "label": "SMTP username", "hint": "The sending account / login." },
+    "smtp_from": { "label": "From address", "hint": "e.g. Siftlode <addr@gmail.com>. Defaults to the username." },
+    "smtp_password": { "label": "SMTP password", "hint": "App password. Stored encrypted; write-only — it's never shown back." }
+  },
+  "save": "Save",
+  "saved": "Saved",
+  "saveFailed": "Couldn't save",
+  "reset": "Reset",
+  "resetHint": "Remove this override and fall back to the file/env default.",
+  "resetDone": "Reset to default",
+  "usingDefault": "using default",
+  "overridden": "overridden in DB",
+  "secretSet": "•••••••• stored",
+  "secretEnv": "using env value",
+  "secretUnset": "not set",
+  "secretsDisabled": "Set TOKEN_ENCRYPTION_KEY to store secrets in the database.",
+  "testEmail": "Send test email",
+  "testEmailHint": "Send a test email to your own address using the settings above, to confirm they work.",
+  "testSent": "Test email sent to {{to}}",
+  "testFailed": "Test email failed — check the settings"
+}

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

@@ -4,6 +4,7 @@
   "channelManager": "Channel manager",
   "usageStats": "Usage & stats",
   "scheduler": "Scheduler",
+  "configuration": "Configuration",
   "scope": {
     "label": "Feed source",
     "my": "Mine",
@@ -18,6 +19,7 @@
     "playlists": "Playlists",
     "stats": "Stats",
     "scheduler": "Scheduler",
+    "config": "Configuration",
     "settings": "Settings",
     "about": "About",
     "signOut": "Sign out",