feat(loadfile): request directory contents via new iframe API

[tavateva]

Closes #9.

Depends on ouroboros #114 (merged) which added the request/response iframe API.

Problem

Since ouroboros #106, send-directory-contents arrives with nodes: {}. The plugin's LoadFile modal renders an empty list — Object.values(directoryData.data.nodes).map(...) sees nothing.

What changed

Wire up the new API in src/App/index.tsx:

• On each send-directory-contents (root change), dispatch a request-directory-contents with recursive: true and a monotonic requestId (`loadfile-N`).
• Track the active requestId in a ref; drop late responses whose id doesn't match, so a response for a previous root can't clobber the current tree.
• On send-directory-contents-response, merge data.nodes back into directoryData so LoadFile's existing tree renderer works again without changes.
• Guard the merge with a data.path === current directoryPath check — belt-and-suspenders against the same out-of-order case.
• Handle error.code:
  • denied / not-found / internal → empty tree (matches the existing "no files" rendering).
  • internal also logs via console.error.
  • limit with truncated: true → render the partial nodes the response carries. The truncation hint UI is deferred as a follow-up so this diff stays surgical.

LoadFile itself is unchanged.

Verify

• npx tsc -b — clean (typecheck passes).
• npm run build — clean.
• npm run lint — pre-existing lint errors in the repo (288, mostly @ts-nocheck and no-explicit-any in NG viewer code, unrelated to this change). No new errors introduced (identical count on `main` vs. this branch).

Manual UI smoke against a running Ouroboros not performed here — no display on the automation host. Reviewer checklist for a live test:

1. Open Ouroboros, load the neuroglancer plugin, and Open Folder → a small directory.
2. LoadFile modal should populate with the folder tree.
3. Change root folder → tree updates to the new root.
4. Rapid root changes → no stale-tree flicker (thanks to requestId matching).
5. Load against a folder large enough to hit the enumeration cap → partial tree renders, no crash.

Not changed

• Message-handling structure elsewhere (`send-neuroglancer-json`, `read-file-response`, save flow) is untouched.
• Autoseg plugin doesn't need this — it doesn't use parent-window directory messaging.

Followup

If desired, surface `error.truncated: true` in the LoadFile UI (e.g., "showing partial tree") — separate small PR.