Masked translation round-trip; named translator keys

- pagerite/masking.py: technical spans (code, URLs, {placeholders}, attrs,
  footnote/link labels, container names, HTML tags) become numbered sentinels
  for the LLM round trip; results are restored by number and rejected when a
  sentinel is mangled (skipped for the rest of the run, stays pending).
  Chunks with no prose left after masking are never dispatched.
- Data.translate_key -> translate_keys dict (key -> name); the first key is
  generated at bootstrap, result transactions record the key as user=, and
  startup logs the service URL(s) via translate.log_service_urls.
- Fix /_translate proxying through the Vite dev server (missing slash).

frontend/vite.config.js

@@ -16,7 +16,7 @@ const CONTENT_PROXY = '^(?!/_|/@|/src|/node_modules|/__).*$'
 // https://vite.dev/config/
 export default defineConfig({
   plugins: [
-    fastapiVue({ paths: ["/_api", "/_f", "/_themes", "/_fonts", "/_a"] }),
+    fastapiVue({ paths: ["/_api", "/_f", "/_themes", "/_fonts", "/_a", "/_translate"] }),
     vue(),
     vueDevTools(),
   ],