Add target FPS mode to all VFI models and remove concat preview

Add source_fps/target_fps inputs to all 4 Interpolate and Segment nodes
(BIM, EMA, SGM, GIMM). When target_fps > 0, auto-computes optimal
power-of-2 oversample, runs existing recursive t=0.5 interpolation,
then selects frames at target timestamps. Handles downsampling (no
model calls), same-fps passthrough, and high ratios (e.g. 3→30fps).
Segment boundary logic uses global index computation for gap-free
stitching. When target_fps=0, existing multiplier behavior is preserved.

Remove video preview from TweenConcatVideos: drop preview input,
delete web/js/tween_preview.js, remove WEB_DIRECTORY from __init__.py.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

__init__.py

@@ -52,8 +52,6 @@ from .nodes import (
     LoadGIMMVFIModel, GIMMVFIInterpolate, GIMMVFISegmentInterpolate,
 )
 
-WEB_DIRECTORY = "./web"
-
 NODE_CLASS_MAPPINGS = {
     "LoadBIMVFIModel": LoadBIMVFIModel,
     "BIMVFIInterpolate": BIMVFIInterpolate,