feat: move frame paths to left column with strength + logic index switch

Each frame path row (start/middle/end) now has:
- path input with preview
- strength float (default 1.0)
- switch linked to the corresponding logic index bit

Switches and logic index are bidirectionally synced.
end_frame → logic index → switches mirror chain preserved.

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

utils.py

@@ -47,8 +47,11 @@ DEFAULTS = {
     "reference switch": 1,
     "video file path": "",
     "start frame path": "",
+    "start frame strength": 1.0,
     "middle frame path": "",
+    "middle frame strength": 1.0,
     "end frame path": "",
+    "end frame strength": 1.0,
 
     # --- LoRAs (name as STRING, strength as FLOAT) ---
     "lora 1 high": "",