Fix performance regressions and remove dead code

[Copilot]

Several performance issues and dead code paths identified across the video editor frontend.

Composition: fix runaway Redux dispatches

previousTime.current was never updated after dispatching setCurrentTime, so the threshold check always passed after the first crossing — dispatching on every frame (~30/s instead of only on meaningful changes).

dispatch(setCurrentTime(currentTimeInSeconds));
previousTime.current = currentTimeInSeconds; // was missing

Also moved fps declaration above useEffect (was referenced before declaration) and removed 6 unused imports.

VideoTimeline: prevent unnecessary re-renders

Split useAppSelector((state) => state.projectState) into individual field selectors. The component was re-rendering on every currentTime change (30fps) despite only needing mediaFiles, activeElement, activeElementIndex, and timelineZoom.

FfmpegRender: remove double file loading + dead code

• A Promise.all loaded every media file into FFmpeg, but the results were never used — the loop below re-loaded them individually via wroteFiles cache. Removed the unused parallel load.
• Removed shadowed wroteFiles declaration (outer one was dead).
• Replaced console.log('No media files to render') with toast.error() for user visibility.

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