Scope git commit to current diagram + fix complete JSON export/import

- Git commit: "Current diagram only" checkbox stages only that diagram's
  JSON file instead of all diagrams
- JSON export now includes: waypoints, twisted_pair, twist_pitch, shielded,
  show_size_label, bundle_id, bundle_label, sheet_id, sheets, wire_bundles
- JSON import reconstructs sheets and bundles with remapped IDs so restored
  diagrams are exact copies

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/js/api.js

@@ -63,7 +63,7 @@ const api = {
   git: {
     status:  ()                             => apiFetch("/git/status"),
     log:     ()                             => apiFetch("/git/log"),
-    commit:  (message)                      => apiFetch("/git/commit",  { method: "POST", headers: { "Content-Type": "application/json" }, body: JSON.stringify({ message }) }),
+    commit:  (message, diagram_id)          => apiFetch("/git/commit",  { method: "POST", headers: { "Content-Type": "application/json" }, body: JSON.stringify({ message, diagram_id: diagram_id ?? null }) }),
     push:    ()                             => apiFetch("/git/push",    { method: "POST" }),
     history: (hash)                         => apiFetch(`/git/history/${hash}`),
     restore: (commit_hash, filepath, name)  => apiFetch("/git/restore", { method: "POST", headers: { "Content-Type": "application/json" }, body: JSON.stringify({ commit_hash, filepath, name }) }),