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>

server/lib/scope.js

@@ -289,12 +289,18 @@ function audienceIncludes(audience, user) {
 
 /**
  * A non-admin's own folder, `<AGENTHUB_USERS_ROOT>/<username>`, or null (no
- * root set, or an admin). Whatever the method: an analyst reads every user's
+ * root set, or an admin). Under S3 a missing variable falls back to
+ * server.conf's `USERS_ROOT` — the folder the member's Linux account lives in —
+ * so a settings file without it can never open the host's whole disk to them.
+ * Whatever the method: an analyst reads every user's
  * ROWS, but a path named in a request is read with the dashboard's own rights
  * (CAP_DAC_READ_SEARCH under S3), so an analyst's GET is held to it too.
  */
 function ownFolder(req) {
-  const root = (process.env.AGENTHUB_USERS_ROOT || "").trim();
+  const linux = require("./linux-accounts");
+  const root =
+    (process.env.AGENTHUB_USERS_ROOT || "").trim() ||
+    (linux.s3Enabled() ? linux.serverConf().usersRoot : "");
   if (!req?.user || req.user.role === "admin" || !root) return null;
   const path = require("node:path");
   return path.join(path.resolve(root), req.user.username);