Add flatfield correction with BaSiCPy integration

[hongquanli]

Summary

• Add flatfield correction module using BaSiCPy for illumination correction
• Two modes: Calculate from tiles or Load from .npy file
• Per-channel flatfield and optional darkfield correction
• Constant darkfield per channel (physically accurate for uniform dark current)

Features

• Calculate: Sample ~50 tiles and compute flatfield using BaSiCPy
• Load: Drag-and-drop .npy flatfield files
• Auto-save: Calculated flatfield saved as {input}_flatfield.npy
• Auto-load: Existing flatfield loaded when input file is dropped
• View: Matplotlib visualization with colorbars (flatfield + darkfield)
• Clear: Reset flatfield state

UI Changes

• New "Flatfield Correction" section between Preview and Settings
• Enable/disable checkbox (default: enabled)
• Radio buttons for Calculate vs Load mode
• Include darkfield correction option
• Status label showing channel count
• View and Clear buttons

Test plan

• Drop OME-TIFF file and verify auto-load of existing flatfield
• Calculate flatfield with darkfield enabled
• View flatfield/darkfield in matplotlib (check constant darkfield values)
• Run stitching with flatfield correction enabled
• Clear flatfield and verify state reset

🤖 Generated with Claude Code