Improve selfie capture handoff flow

[reloxx13]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "x" next to an item)

• Documentation update
• Bug fix
• New feature
• Other, please explain:

What changes did you make? (Give an overview)

This updates the selfie-mode capture handoff so the user keeps visual continuity between taking a photo and seeing the processed result.

It keeps the loader stage available in selfie mode, freezes the live preview into the loader/background during capture and upload, and resets that state cleanly when the booth returns to the start screen.

On the upload path it also improves the selfie form/button markup, adds translated success and error messages, and hardens server-side image handling for EXIF-less environments and non-JPEG source uploads.

Is there anything you'd like reviewers to focus on?

Please focus on the selfie-mode flow:

• selecting or capturing a selfie on a mobile device
• the handoff from preview to loader/result state
• uploads from image sources that are not already JPEGs

AI used to create this Pull Request?

AI assistance was used to help draft the JS/PHP changes and run local verification. The resulting behavior and the final diff were reviewed manually.