Restructure into multi-file architecture

Split monolithic symlink.py into modular components:
- config.py: Constants and configuration
- core/: Models, database, blender, manager
- ui/: Main window and widgets

New features included:
- Cross-dissolve transitions with multiple blend methods
- Alpha blend, Optical Flow, and RIFE (AI) interpolation
- Per-folder trim settings with start/end frame control
- Per-transition asymmetric overlap settings
- Folder type overrides (Main/Transition)
- Dual destination folders (sequence + transitions)
- WebP lossless output with compression method setting
- Video and image sequence preview with zoom/pan
- Session resume from destination folder

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config.py

@@ -0,0 +1,10 @@
+"""Configuration constants for Video Montage Linker."""
+
+from pathlib import Path
+
+# Supported file extensions
+SUPPORTED_EXTENSIONS = ('.png', '.webp', '.jpg', '.jpeg')
+VIDEO_EXTENSIONS = ('.mp4', '.webm', '.mkv', '.avi', '.mov', '.wmv', '.flv', '.m4v')
+
+# Database path
+DB_PATH = Path.home() / '.config' / 'video-montage-linker' / 'symlinks.db'