Merge origin/main (#984, the canvas shell) into the cleanup branch

Conflicts resolved in 16 files. Where main's new shell, classic page and
tests depend on a wire format or a signature the cleanup had changed, main
wins; where main only extended code the cleanup had restructured, main's
change is carried over into the new structure:

- MCP HTTP server: the poll response keeps main's separate export fields
  (exportFormat, exportXml, exportOptions, exportId with a random id)
  instead of the cleanup's pendingExport object; the classic preview page
  is main's version
- MCP index: start_session keeps the cleanup's structure and gains main's
  BROWSER=none, previewUrl and same-origin note; get_selection is written
  in the registerTool style; create_new_diagram keeps the cleanup's
  helpers with main's comment
- history: addHistory takes main's source parameter and returns the index
  again (main's signature)
- new-diagram: the cleanup's prepareNewModel split plus main's page ids
  for diagrams without one
- diagram context: one ExportResult type (main's, with optional fields)
  for both the tagged exports and main's requestExport; the cleanup's
  single export resolver stays
- versions store and version-text test: main's VersionItem base type and
  describeChangeSummary rename, without the legacy history conversion the
  cleanup removed
- dictionaries: main's shell section added; openFileHint stays removed
  (unused on main too)
- tests: the server wiring test keeps the SDK client and gains main's
  session cases; the HTTP server test uses main's export fields; the DOM
  polyfill comes from tests/setup.ts
- package.json: main's postcss kept, the cleanup's removals kept; lockfile
  regenerated

package.json

@@ -124,6 +124,7 @@
         "husky": "^9.1.7",
         "jsdom": "^27.4.0",
         "lint-staged": "^16.2.7",
+        "postcss": "^8.5.6",
         "tailwindcss": "^4",
         "typescript": "^5",
         "vite-tsconfig-paths": "^6.0.3",