feat(client): bring back New folder, icon-only — and close the hole it opened

The control was hidden and its `canCreateDir` left computed and unused. It is
back in the Files toolbar as an icon: the toolbar already carries one labelled
primary action, and a second beside it competes for the width the breadcrumb
trail needs. The name is in `title` *and* `aria-label` — a title is invisible
to a screen reader on a button with no text, so an icon-only control without
both is simply unnamed for anyone not reading with their eyes.

Its gate changes. It required `isNodeAdmin`, which contradicted the node's own
rule — "making a directory is not a privileged act; a member who can add a file
can organise where it goes" — and hid the control from everyone who could have
used it. It now follows the Upload button: a writable root, and not at the top
of a group, where the level is the set of roots rather than a directory on
anyone's disk.

Restoring it surfaced a real gap. `_do_dir_create` never learned about RO/RW:
`_do_file_upload` gained the `writable` check with the model and this one did
not, so a member refused a file in a published library could still leave empty
directories all through it, and could write to a drive mid-eject. Read-only has
to mean read-only for every way of writing, not just for files.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pvMdvLBG92jyhvD5pD6us

1 files changed, 22 insertions, 0 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
index 61458f2..0e2d418 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
@@ -1595,6 +1595,28 @@ class WebRTCPeerSession:
                         "detail": "Choose a folder to create this in"})
             return
 
+        # Read-only means read-only, and creating a folder writes to the
+        # operator's disk. `_do_file_upload` gained this check with the RO/RW
+        # model and this one did not — so a member could not add a file to a
+        # published library but could still leave empty directories in it.
+        owner = roots.split(parent_rel)
+        if owner is None:
+            self._send({"type": "error", "detail": "Invalid directory"})
+            return
+        parent_root, _tail = owner
+        if not parent_root.writable:
+            self._send({"type": "error",
+                        "detail": f"Directory '{parent_root.name}' is read-only",
+                        "code": "root_read_only"})
+            self._audit("dir_create_refused", parent_rel[:64])
+            return
+        if not parent_root.available:
+            self._send({"type": "error",
+                        "detail": f"Directory '{parent_root.name}' is "
+                                  f"currently unavailable",
+                        "code": "root_unavailable"})
+            return
+
         parent = safe_subdir(roots, parent_rel)
         if parent is None or not parent.is_dir():
             self._send({"type": "error", "detail": "Invalid directory"})