Add a way to save recorded/exported Spark videos to Photos

[knotbin]

Allow users to save videos they create in Spark to their phone's photo library.

Summary

Minimum scope could be videos recorded in-app. It may also make sense to support any Spark video that has been rendered locally, including edited gallery uploads.

Motivation

Right now we already generate local MP4s during the recording/editor flow, but the only next steps are posting the result or returning to review. There is no explicit way to keep a copy in Photos for reuse or sharing elsewhere.

Context

• recorded clips are stitched into a temporary spark_recording_<timestamp>.mp4
• edited videos are exported to a temporary spark_edited_<timestamp>.mp4
• those results currently flow into story posting or VideoReviewRoute, not a save-to-device path
• the app already uses photo_manager for media library access, and the current dependency includes PhotoManager.editor.saveVideo(...)
• iOS currently has photo-library read usage text, but a write/save flow will likely need its permission story checked as part of this work

Proposed direction

• add a Save to Photos or Export to Photos action somewhere in the post-record / post-edit flow
• decide whether the action should ship first for recorded-in-app videos only or for all locally rendered Spark videos
• show clear success/failure feedback
• validate iOS and Android permission behavior

Potentially related code

• lib/src/core/pro_video_editor/pro_video_editor_repository_impl.dart
• lib/src/core/pro_video_editor/ui/video_editor_grounded_page.dart
• lib/src/features/posting/ui/pages/recording_page.dart
• lib/src/features/posting/ui/pages/video_review_page.dart
• lib/src/features/posting/ui/pages/media_picker_page.dart
• ios/Runner/Info.plist
• android/app/src/main/AndroidManifest.xml