Auth via paskia-js: profile() dialog, apiFetch/apiJson in admin components

- Banner-corner auth link is now a button opening paskia-js's profile()
  dialog (handles login too); auth re-probed when the dialog closes.
- Editor/analytics components call /_api via apiFetch/apiJson: an
  expired session opens the login dialog and the request retries.
- pagerite.js: task-checkbox toggle uses apiJson (explicit edit attempt,
  reverts on any failure incl. cancelled login); auth probes use
  fetchJson (never a dialog); page-cache/navigation stay on plain fetch.
- Add the paskia npm dependency; document the convention.

frontend/package.json

@@ -20,6 +20,7 @@
     "codemirror": "^6.0.2",
     "country-flag-icons": "^1.6.20",
     "overlayscrollbars": "^2.16.0",
+    "paskia": "^2.1.0",
     "pinia": "^4.0.3",
     "transliteration": "^2.6.1",
     "vue": "^3.5.26",