Lemenzo Gallery v1.21 — Editor modernization and performance

- Photo editor redesigned around a modern bottom-tool workflow: Adjust, Crop, Filters and Markup.
- Sticky non-destructive save actions remain visible while editing.
- LemenzoOS black/navy/blue palette used throughout editor surfaces; no grey-heavy redesign.
- Photo preview edge reduced to 1600 px for interactive editing only; full-resolution export remains unchanged.
- Preview rendering moved off the UI thread, debounced and generation-coalesced to prevent slider jank and stale renders.
- Video editor redesigned with full-height preview, play/pause, trim card, transform tools and sticky save action.
- Video metadata duration probing moved off the UI thread.
- Video trim scrubbers seek the preview while dragging.
- Existing lossless stream-copy trim/mute/rotation exporter retained; no unnecessary video re-encode introduced.
- Existing SystemInsets handling retained for status bar / display cutout safety.