Replace put_flash with inline feedback for form saves:
- Media library: metadata save shows "Saved" checkmark
- Product show: storefront controls save shows "Saved" checkmark
- Newsletter campaign form: draft save shows "Saved" checkmark
Page-level outcomes (uploads, deletes, async operations) remain as
flash/banner messages — these are the correct pattern for non-form
actions.
Completes Task 4 of notification overhaul.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>