fix(webui): require token_issue_secret for LAN access with frontend auth

When host is set to 0.0.0.0, the gateway now enforces that either token
or token_issue_secret must be configured — it refuses to start otherwise.

Bootstrap endpoint behavior:
- token_issue_secret configured: always validate regardless of source IP
  (handles reverse-proxy scenarios where all connections appear as localhost)
- No secret: only localhost can bootstrap (local dev mode)

The frontend shows an authentication form when bootstrap returns 401/403,
persists the secret in localStorage, and retries automatically on reload.

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

@@ -9,6 +9,18 @@
       "title": "Couldn't reach nanobot",
       "gatewayHint": "Make sure the gateway is running (`nanobot gateway`) and that this page is open on the same machine."
     },
+    "auth": {
+      "title": "Authentication required",
+      "hint": "Enter the secret configured as tokenIssueSecret in your gateway config.",
+      "placeholder": "Password",
+      "submit": "Connect",
+      "invalid": "Invalid password. Try again."
+    },
+    "account": {
+      "section": "Account",
+      "logoutHint": "Disconnect this browser from the gateway.",
+      "logout": "Sign out"
+    },
     "documentTitle": {
       "base": "nanobot",
       "chat": "{{title}} · nanobot"