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/index.html

@@ -430,7 +430,12 @@
 
       <!-- Commit section -->
       <div class="git-section">
-        <label class="git-section-title">Commit all diagrams</label>
+        <div style="display:flex;align-items:center;justify-content:space-between">
+          <label class="git-section-title">Commit diagrams</label>
+          <label style="font-size:11px;color:#8899bb;display:flex;align-items:center;gap:5px;cursor:pointer">
+            <input type="checkbox" id="git-scope-current"> Current diagram only
+          </label>
+        </div>
         <div style="display:flex;gap:6px;align-items:flex-start">
           <textarea id="git-commit-msg" placeholder="Describe what changed…" rows="2"
             style="flex:1;resize:vertical;font-size:12px;padding:5px;background:#0d0d20;color:#ccd;border:1px solid #2a2a44;border-radius:4px"></textarea>