feat(ai-settings): add per-card "Save and test" button

The single global "Save endpoints" button sat far below the fold and the
per-card "Test endpoint" button probed the server-stored settings, so it
ignored unsaved form edits. Replace each endpoint card's "Test endpoint"
button with a combined "Save and test" button that persists the whole form
first and only runs the card's connection probe on a successful save; the
global "Save endpoints" button is kept for save-only.

- Add handleSaveAndTest: save (rethrows on failure) then probe; skip the
  test if the save fails (the mutation already surfaces the error).
- Add savingTestCapability state so only the clicked card spins during the
  shared save while all save controls stay disabled (no concurrent saves).
- Reset the previous probe result when a new save+test starts.
- Add the "Save and test" en-US translation key.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/public/locales/en-US/translation.json

@@ -1205,6 +1205,7 @@
   "Transcribe as you speak, cutting on pauses": "Transcribe as you speak, cutting on pauses",
   "Voice dictation is not available yet.": "Voice dictation is not available yet.",
   "Test endpoint": "Test endpoint",
+  "Save and test": "Save and test",
   "Save endpoints": "Save endpoints",
   "Configured and enabled": "Configured and enabled",
   "Configured but disabled": "Configured but disabled",