Fix Windows preview navigation and CodeMirror notice

[theincrediblewwz]

Summary

• reserve the top-right native preview scrollbar gutter from window dragging
• add a configurable source-to-preview shortcut with block/line-aware positioning
• include the complete CodeMirror MIT notice in source and Windows portable packages
• extend Electron and package smoke coverage

Verification

• Windows full unit suite: 79 files / 511 tests passed
• Windows production build passed
• Electron smoke passed, including F8 persistence, 120-line remote navigation, and physical top-thumb dragging without moving BrowserWindow
• public-source gate READY; CI config contract passed
• Windows portable smoke passed

Release v0.2.0-alpha.1 will only replace the old release after all same-commit platform assets are rebuilt and verified.