replace Imgur with GitHub Release Assets for screenshot hosting (AppI…

…mage#3573)

* replace Imgur with GitHub Release Assets for screenshot hosting

Fixes AppImage#3548

## Problem
Imgur API returns "Too Many Requests" errors (code 1025), blocking
successfully tested applications from being merged.

## Solution
Replace Imgur with GitHub Release Assets:
- Screenshots are uploaded to a draft release named 'ci-screenshots'
- Inline images displayed in PR comments
- Works for PRs from external contributors using workflow_run trigger

* sanitize filenames for artifact upload

- Add step to sanitize screenshot filenames before upload
- Remove invalid characters (: * ? < > |) from app names
- Fixes error with apps like 'fre:ac' containing colons
- Update publish workflow to handle flat file structure

* ci: fix artifact path and publish release; guard workflow to upstream repo

Update worker.sh