feat(server-mode): native-like folder browser, a developer kept to their own folder, no password minimum

- Folder browser (the Browse fallback for a remote browser): back/forward/up,
  a clickable breadcrumb that becomes a path field (Ctrl+L), Places (Home,
  the user's workspaces, recent picks), a filter, Name/Modified columns,
  Show hidden and New folder. A click selects; a double-click or Enter
  opens a folder or picks a file; Backspace goes up; typing jumps to a name.
- GET /api/lanes/browse: `hidden=1` (never the dot entries at the top of a
  non-admin's own folder), each entry's `mtime`, the answer's `home`.
- POST /api/lanes/browse/mkdir: one plain folder name inside an existing
  folder; a non-admin only in their own folder, none in server mode without
  one; under S3 a member's folder is made as the member.
- scope.ownFolder falls back to server.conf's USERS_ROOT under S3, so a
  settings file without AGENTHUB_USERS_ROOT no longer opens the host's home
  to every developer.
- Passwords: any non-empty password is accepted (no 8-character minimum).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

client/src/i18n/locales/en/users.json

@@ -25,7 +25,7 @@
   "username": "Username",
   "usernameHint": "Letters, digits, '.', '_' or '-'",
   "password": "Password",
-  "passwordHint": "At least 8 characters",
+  "passwordHint": "No minimum length",
   "newPassword": "New password",
   "changeRole": "Change role",
   "disable": "Disable",
@@ -64,7 +64,7 @@
     "ESELF": "You cannot change your own role or disable yourself.",
     "EEXISTS": "That username is taken.",
     "EBADUSER": "Username: letters, digits, '.', '_' or '-' (up to 64).",
-    "EBADPASSWORD": "Password: at least 8 characters."
+    "EBADPASSWORD": "Password cannot be empty."
   },
   "readOnly": "Analysts can view accounts but not change them.",
   "auditTitle": "Audit log",