notes(folders): allow drag-drop of notes onto shared folders (edit perm)

User report (AR): "shared folders also appear here — the user should
be able to move notes wherever they want." The folders rail already
lists shared-with-me folders, but only OWN folder rows registered as
@dnd-kit drop targets. Recipients with edit permission could see a
shared folder but couldn't drop a note onto it.

Changes:
- folders-rail.tsx: extend the inline `DroppableRow` to accept an
  optional `ownerId`. Wrap each shared-folder row in a DroppableRow
  when `sf.myPermission === "edit"`, passing the owning user's id;
  read-only viewers stay non-droppable. The droppable id is namespaced
  with `-shared-<ownerId>` so it never collides with the owner-side
  row for the same folder id.
- notes.tsx: extend `sharedFolderBucketRef` to carry the bucket's
  `ownerId` (sourced from `data.folder.ownerId`). In `handleDragEnd`,
  read `o.ownerId` from the drop data:
    • undefined  → own folder / Unfiled. Allow only own notes.
    • number     → shared folder. Allow only when the source note
                   came from sharedBucket and `sharedBucket.ownerId
                   === o.ownerId`.
  This mirrors the server-side rule that an editor may move notes
  only between folders owned by the same owner — cross-owner drops
  are blocked client-side so we never round-trip a 400.

Out of scope:
- Cross-owner moves (would require a copy/share flow, not supported
  by the server today).
- Drop targets on shared folders inside the FoldersBrowser tile grid
  (the rail covers the user's reported screenshot; tiles can follow).

artifacts/tx-os/src/components/notes/folders-rail.tsx

@@ -84,10 +84,8 @@ function DroppableRow({
   children,
 }: {
   target: "unfiled" | number;
-  // When set, the drop target belongs to a shared folder owned by this
-  // user id. handleDragEnd uses this to enforce the "same-owner only"
-  // rule the server applies to editors. Leave undefined for own folders
-  // and the Unfiled bucket.
+  // Set only for shared-folder rows, so handleDragEnd can enforce the
+  // server's same-owner rule for editors.
   ownerId?: number;
   className?: string;
   testId?: string;
@@ -515,9 +513,6 @@ export function FoldersRail({
               },
               lang,
             );
-            // Editors can drop notes from this owner's namespace into
-            // any of their shared folders (server enforces same-owner).
-            // Read-only viewers stay non-droppable.
             const canDrop = sf.myPermission === "edit";
             const rowInner = (
               <div

artifacts/tx-os/src/pages/notes.tsx

@@ -349,9 +349,6 @@ export default function NotesPage() {
         | {
             type?: string;
             target?: "unfiled" | number;
-            // Owner of the target droppable. Set ONLY for shared-folder
-            // rows; absent (undefined) on own folders + the Unfiled bucket
-            // — those targets implicitly belong to the current user.
             ownerId?: number;
             noteId?: number;
             folderId?: number | null;
@@ -370,14 +367,7 @@ export default function NotesPage() {
       if (!isOwn && !isEditableShared) return;
 
       if (o?.type === "folder-drop" && o.target !== undefined) {
-        // The server enforces: an editor may only move a note BETWEEN
-        // folders owned by the same owner. Mirror that constraint
-        // client-side so cross-owner drops never round-trip:
-        //   • Target with no ownerId  = own folder / Unfiled. Allow only
-        //     for the user's OWN notes.
-        //   • Target with ownerId     = a shared folder we can edit.
-        //     Allow only when the source note also lives in that owner's
-        //     namespace (i.e. came from sharedBucket with the same owner).
+        // Enforce same-owner rule client-side (server also validates).
         if (o.ownerId === undefined) {
           if (!isOwn) return;
         } else {