export/import carries notes + texts; new Import button

- exportJson now includes per-floor notes and texts arrays alongside route/pulls; bumps a version field for forward-compat
- new importJson: file picker, parses JSON, resolves dungeon by id against current build, replaces this dungeon's pins on every floor present in the file
- Import button next to Export in the toolbar

web/index.html

@@ -36,7 +36,8 @@
       <button id="undo" title="Undo (⌘Z)">Undo</button>
       <button id="clear" title="Clear current floor">Clear</button>
       <button id="share">Share</button>
-      <button id="export">Export JSON</button>
+      <button id="export" title="Download this dungeon's routes/pulls/notes/labels as JSON">Export</button>
+      <button id="import" title="Load a previously exported route file">Import</button>
     </div>
   </header>