feat(setup): install-wizard UI (epic 6c)

- Multi-step first-run wizard (intro → admin → Google → SMTP → finish), shown by App while the
  instance reports configured=false; the one-time token comes from the setup URL (?token=) and is
  sent as X-Setup-Token on every step. Google/SMTP steps appear only when secrets can be stored.
- App holds the 'me' query until setup status is known (so it doesn't 503 against the setup lock);
  a missing/invalid token shows a 'use the link from the logs' screen. EN/HU/DE.

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

@@ -0,0 +1,50 @@
+{
+  "intro": {
+    "title": "Welcome — let's set up Siftlode",
+    "body": "A few quick steps to get your instance ready: create your admin account, and optionally connect Google sign-in and email. You can change all of this later from the admin settings."
+  },
+  "admin": {
+    "title": "Create your admin account",
+    "desc": "This is the first account and an administrator. You'll sign in with this email and password once setup is done.",
+    "email": "Email address",
+    "password": "Password (min. {{n}} characters)",
+    "invalidEmail": "Enter a valid email address.",
+    "shortPassword": "Password must be at least {{n}} characters."
+  },
+  "google": {
+    "title": "Sign in with Google (optional)",
+    "desc": "Paste your Google OAuth client ID and secret to enable Google sign-in and YouTube access. Leave both empty to skip — email+password works without it.",
+    "clientId": "Google client ID",
+    "clientSecret": "Google client secret",
+    "bothRequired": "Enter both the client ID and secret, or leave both empty to skip.",
+    "skipHint": "Leave empty to set this up later from the admin Configuration page."
+  },
+  "smtp": {
+    "title": "Email / SMTP (optional)",
+    "desc": "Configure an SMTP server so Siftlode can send verification, reset and notification emails. Leave the host empty to skip — without email, registrations are auto-approved by the admin instead.",
+    "host": "SMTP host (e.g. smtp.gmail.com)",
+    "port": "Port",
+    "user": "Username",
+    "from": "From address (optional)",
+    "password": "SMTP password",
+    "testTo": "Send a test email to…",
+    "test": "Send test",
+    "testOk": "Test email sent — check the inbox.",
+    "testFail": "Couldn't send — check the settings.",
+    "skipHint": "Leave empty to set this up later from the admin Configuration page."
+  },
+  "finish": {
+    "title": "All set!",
+    "body": "Click Finish to complete setup. The wizard will disappear and you'll be taken to the sign-in page — log in with the admin account you just created."
+  },
+  "nav": {
+    "back": "Back",
+    "next": "Next",
+    "finish": "Finish"
+  },
+  "token": {
+    "title": "Setup link needed",
+    "body": "Open the setup wizard using the link printed in the container logs at first start (it looks like …/setup?token=…). It's required so only the operator can configure this instance."
+  },
+  "genericError": "Something went wrong — please try again."
+}