Improve README clarity and add defaults to controls

- Expand Controls intro with two-panel layout description
- Add brush size range/default and mask alpha range/default to tables
- Split 'Starting-point shortcuts' (Load Prev Mask, Auto Segment) from mask editing table
- Trim 'How it works' mask drawing section to remove implementation internals
- Enable optical_flow_config_file in config.example.yaml

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

config/config.example.yaml

@@ -19,7 +19,7 @@ max_frames: 100
 data_dir: data/filtered_data
 out_dir: data/annotation_results
 clips_file: config/clips.txt
-# optical_flow_config_file: config/optical_flow_config.yaml
+optical_flow_config_file: config/optical_flow_config.yaml
 
 questions:
   - section: River