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

2 files changed, 63 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"})
diff --git a/packages/meshbay-node/tests/test_root_writable_policy.py b/packages/meshbay-node/tests/test_root_writable_policy.py
index d7f2666..7eb75fd 100644
--- a/packages/meshbay-node/tests/test_root_writable_policy.py
+++ b/packages/meshbay-node/tests/test_root_writable_policy.py
@@ -108,6 +108,47 @@ async def test_read_only_binds_the_operator_too(tmp_path):
     assert refusal and refusal[0].get("code") == "root_read_only"
 
 
+async def test_a_member_cannot_create_a_folder_in_a_read_only_root(tmp_path):
+    """
+    Read-only has to mean read-only for every way of writing, not just for
+    files. `_do_file_upload` gained this check with the RO/RW model and
+    `_do_dir_create` did not, so a member refused a file in a published library
+    could still leave empty directories all through it.
+
+    Creating a folder stays unprivileged — the node's own words: "a member who
+    can add a file can organise where it goes". What changed is that it now
+    requires the same root to be writable that adding the file would have.
+    """
+    session = _session(tmp_path, "member-1", writable=False)
+    session._do_dir_create({"dir": "shared", "name": "New folder"})
+
+    refusal = [m for m in session.sent if m.get("type") == "error"]
+    assert refusal and refusal[0].get("code") == "root_read_only"
+    assert not (tmp_path / "shared" / "New folder").exists()
+
+
+async def test_a_member_can_create_a_folder_in_a_writable_root(tmp_path):
+    """The counter-property: it must stay unprivileged where it is allowed."""
+    session = _session(tmp_path, "member-1", writable=True)
+    session._do_dir_create({"dir": "shared", "name": "New folder"})
+
+    assert not [m for m in session.sent if m.get("type") == "error"]
+    assert (tmp_path / "shared" / "New folder").is_dir()
+
+
+async def test_an_ejected_root_refuses_a_new_folder(tmp_path):
+    """Writing to a drive somebody has their hand on, one level up from a file."""
+    session = _session(tmp_path, "member-1", writable=True)
+    roots = session._ctx["roots"]
+    roots.roots[0].ejected = True
+    roots.roots[0].available = False
+
+    session._do_dir_create({"dir": "shared", "name": "New folder"})
+    refusal = [m for m in session.sent if m.get("type") == "error"]
+    assert refusal and refusal[0].get("code") == "root_unavailable"
+    assert not (tmp_path / "shared" / "New folder").exists()
+
+
 # ── Signed, or it is a suggestion ────────────────────────────────────────────
 
 def _capture_challenges(session) -> list[tuple[str, str]]: