0.18.3: grouped clip tracks render, playhead is a snap magnet

- Fix mask-preview / frame-export dropping any group whose children
  are clip tracks. render.ts's old group branch only pulled Layer
  children out of group.childIds; new drawTrackGroup walks both
  layer AND clip-track ids and still applies group-local masks to
  the composite. Same fix mirrored into the HTML export runtime in
  exportPlayer.ts.
- Snapping magnet for the playhead: snapTime now includes
  store.playhead in its candidate list by default. Clip drags, trim
  handles, keyframe drags, marker drags and in/out brackets stick
  to the needle when snap is on. Scrub handlers pass
  ignorePlayhead:true so the needle doesn't stick to its own prior
  position during a scrub.
- Clip trim handles derive the snapped edge first and back out to
  trimStart/trimEnd, so the edge lands exactly on the magnet.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/version.ts

@@ -1,2 +1,2 @@
-export const APP_VERSION = '0.18.2'
+export const APP_VERSION = '0.18.3'
 export const APP_NAME = 'RUWIO'