Fixed: translation failing with a misleading "addon needs to update some data
from translate.google.com" message.

The extension relied on a single Google endpoint with no working fallback. When
that endpoint rate-limited a request, the error was discarded and the extension
fell back to a legacy API path that has not worked for years, blaming a token
problem that was never the real cause.

• Translation now falls back across several endpoints automatically
• HTTP errors are detected properly, with a per-endpoint cooldown instead of
  retrying a server that is throttling
• Error messages now report what actually failed
• Restored the dictionary and synonyms panel in the popup, which had silently
  stopped being filled in
• Optional non-Google fallback services (Lingva, LibreTranslate) can be enabled
  in the options. Disabled by default: nothing is sent to any third-party server
  unless you turn it on and enter an instance address yourself.