feat: rename 'reference image path' to 'start frame path'

Updates DEFAULTS, standard_keys, UI label, timeline known_keys.
Migration auto-renames old key on load.

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

tab_timeline_ng.py

@@ -618,7 +618,7 @@ def _render_preview_fields(item_data: dict):
     known_keys = {
         'sequence_number', 'general_prompt', 'general_negative', 'current_prompt', 'prompt',
         'negative', 'camera', 'flf', 'seed', 'resolutions',
-        'frame_to_skip', 'vace schedule', 'video file path', 'middle frame path', 'end frame path',
+                'frame_to_skip', 'vace schedule', 'video file path', 'middle frame path', 'end frame path', 'start frame path',
     }
     # also skip lora keys
     custom_keys = [