Replace put_flash calls with inline feedback for form saves:
- Email settings: "Now send a test email" after saving
- Settings: from address and signing secret saves
- Page editor: save button shows "Saved" checkmark
Inline feedback appears next to save buttons and auto-clears after
3 seconds. Banners (put_flash) remain for page-level outcomes like
deletions, state changes, and async operations.
Task 3 of notification overhaul. Theme editor skipped as it auto-saves.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
grouped providers by category, added per-provider key validation
with cross-provider detection, friendly delivery error messages,
retryable vs config error distinction, from-address in general
settings, and "Save settings" button to match admin conventions
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
