fix(samba): separate storage-ROOT validation from share-TARGET validation

Live validation caught it: the 'new folder' flow passed the storage root through
sharingResolvePath, which (correctly) refuses the drive root as a share target —
so share creation silently failed. sharingResolveStorageRoot accepts EXACTLY a
registered live root (strictly tighter) and is used only as the new-folder parent.
Regression test asserts both halves.

controller/internal/web/sharing_handlers_test.go

@@ -115,6 +115,43 @@ func TestSharingResolvePath_UniformRefusal(t *testing.T) {
 	}
 }
 
+// The "new folder" flow validates the storage ROOT, which the share-target guard rightly refuses.
+// Regression guard for the live-validation bug: reusing sharingResolvePath there broke share
+// creation entirely (the root is never a valid share TARGET, but is the valid PARENT).
+func TestSharingResolveStorageRoot(t *testing.T) {
+	s, drive := newSharingServer(t)
+
+	// The registered root is accepted here...
+	if got, err := s.sharingResolveStorageRoot(drive); err != nil || got == "" {
+		t.Errorf("a registered storage root must be accepted as a new-folder parent, got %q err=%v", got, err)
+	}
+	// ...even though it is (correctly) refused as a share TARGET.
+	if _, err := s.sharingResolvePath(drive); err == nil {
+		t.Error("the drive root must still be refused as a share target")
+	}
+
+	// Anything that is not EXACTLY a registered live root is refused.
+	for _, bad := range []string{
+		filepath.Join(drive, "media"), // a subdir is not a root
+		filepath.Join(drive, "appdata"),
+		t.TempDir(), // unregistered
+		"relative",
+		"",
+	} {
+		if _, err := s.sharingResolveStorageRoot(bad); err == nil {
+			t.Errorf("%q must not be accepted as a storage root", bad)
+		}
+	}
+
+	// A decommissioned root stops being a valid parent.
+	if err := s.settings.SetDecommissioned(drive, ""); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := s.sharingResolveStorageRoot(drive); err == nil {
+		t.Error("a decommissioned root must not accept new folders")
+	}
+}
+
 // pathWithin must be segment-wise: a sibling directory sharing a name PREFIX is not containment.
 func TestPathWithin_SiblingPrefixIsNotContainment(t *testing.T) {
 	if pathWithin("/mnt/drive-evil/x", "/mnt/drive") {