fix(mobile): retain pinch zoom on inline chat images #33834

Pinching an image directly in the iPhone/iPad chat should retain its zoom and follow the pinch location. The user reports it springs back and scales from the center.

Current-main iPhone simulator reproduction: three inline pinches retained no inline zoom and opened the full-screen preview on release. Full-screen Quick Look already retains zoom and anchors correctly.

Acceptance: pinch inline images around either side, retain scale and position on release and rerender, pan while enlarged, pinch back to fit, and keep ordinary tap-to-preview and chat scrolling working. Cover Markdown images and message attachments.