feat(client): finish card banners and editor flow

[sivert-io]

Card banners support local drag and zoom, crop before Save, animated image and MP4 previews, and local banner files attached to saved presets. Copy card and owl actions open editors. Cancel confirms unsaved changes and resets previews immediately. Preset banner loads are guarded against stale async results.

Chat uses the shared virtualized Gryt UI emoji picker, with current-server custom emoji first. Four real browser tests pass for card editing/cancel/no early upload, custom emoji in chat and card patterns, Unicode search, and message/reaction regressions. The custom emoji pattern test also checks rendered SVG content.

Desktop video posters are decoded in a hidden sandboxed Electron renderer, with no Node integration or preload, denied filesystem/network access, bounded input, and a timeout. Real MP4/WebM tests pass on Windows, macOS, and Linux CI. The Node image worker communicates with this decoder through its parent IPC channel.

Updated @gryt/ui to published 0.39.2-beta.1; npm latest remains 0.39.1. Worker PR #38 requires a human approval. Server PR #268 and beta changelog site PR #267 are merged. No server/client beta has been dispatched.

Poster decoding is not whole-video sanitization or antivirus scanning.

[sivert-io]

Replaced by work that has since landed: the chat emoji picker as #773, video banners and avatars as #774 and #776, and the desktop video decoding as a sandboxed renderer in #775 and #780. The banner crop isn't needed after server#271 stores banners in the card's own shape.