Notes: show "Shared with me" folders on the default screen too

Task #453. Recipient (e.g. yas) couldn't see folders that an owner
(e.g. admin) had shared with them — the server returned them
correctly via /note-folders/shared-with-me, but the rail's
"Shared with me" section was gated on `mode === "rail"`. On the
default Notes screen (My Notes tab + "All notes" selection) the
rail is mounted in `chips-only` mode, so the section was never
rendered. Most recipients only ever saw the default screen, so
shared folders were effectively invisible.

Fix: drop the `mode === "rail"` gate in folders-rail.tsx so the
shared-with-me section renders in both modes. The section already
uses `contents md:block` (so chips inline into the mobile horizontal
strip), and its "Shared with me" label is already `hidden md:block`,
so the rendering stays compact on mobile and labelled on desktop in
both modes.

No backend changes; the existing socket invalidations on
`note-folder-shared` / `note-folder-unshared` already keep the
list fresh in realtime.

Out of scope:
- FoldersBrowser (the grid/list of own folders in the main content
  area) intentionally not extended to also list shared folders;
  the rail entry is sufficient and matches the task's "as a chip
  alongside the own-folder chips" outcome on mobile and
  "Shared with me" sidebar section on desktop.
- No changes to permissions, notifications, or per-note sharing.
- Notes top-bar freeze (Task #452) is untouched.

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

@@ -532,7 +532,7 @@ export function FoldersRail({
         </button>
       )}
 
-      {mode === "rail" && sharedFolders.length > 0 && (
+      {sharedFolders.length > 0 && (
         <div
           className="contents md:block md:pt-3"
           data-testid="shared-with-me-section"