Dismiss chat image previews with browser Back

[hamzamerzic]

Summary

• make chat image previews participate in shell history so Back closes the viewer without leaving the current conversation
• route message images, galleries, sent attachments, and composer attachments through one shared dismissal contract
• keep the viewer's close controls and browser Back on the same history path

Testing

• npm run build
• focused image gallery, attachment, and navigation unit tests
• added a Playwright regression covering Back from an open chat image (left for Docker-backed CI)

[hamzamerzic]

Superseded by the reviewed visual integration in #295, which preserves this change and its authorship. The integration is now moving through the merge queue.