isokit: web Studio (engine as WASM, projects in the browser or a live folder)

- isokit-wasm: wasm-bindgen build of the engine; a project in memory behind
  the same JSON API as `isokit studio`, plus the MCP tools
- isokit-project: Store trait (FsStore folder, MemStore memory) under a
  generic Workspace; Project::from_files; shared Studio API module (api.rs)
  used by serve.rs and the WASM build; web_time for clocks in the browser
- studio.js runs on a pluggable backend (window.IsoKitBackend); no Studio
  keys while a dialog is open
- web/: projects in IndexedDB, new/demo/delete, zip export/import (stored and
  deflated), folder import by drop or picker, folders opened live via File
  System Access with outside edits followed, code editor, page build and
  preview, two tabs in step, isokit.mcp() and WebMCP registration
- tools/build_web.py -> dist/web (static), tools/check_web.py (44 checks)

studio/index.html

@@ -22,7 +22,7 @@ document.documentElement.setAttribute("data-theme",t);})();
   <button type="button" id="redo" title="Redo (Ctrl+Y)" disabled>&#x21B7; Redo</button>
   <span class="st-save" id="save" title="Every change is saved to scene.json at once">Saved</span>
   <span class="st-sep"></span>
-  <a class="st-btn" href="/view" target="_blank" rel="noopener" title="The scene as visitors see it (live)">Preview</a>
+  <a class="st-btn" id="preview" href="/view" target="_blank" rel="noopener" title="The scene as visitors see it (live)">Preview</a>
   <button type="button" id="build" title="Write the self-contained HTML page">Build page</button>
   <button type="button" id="theme" title="Light / dark">&#x25D0;</button>
   <button type="button" id="help" title="Keys and tips">?</button>