feat: drag-and-drop folder to sidebar to create project

[gcheng001]

Summary

Allow users to drag a folder from Finder directly onto the ChatListPanel sidebar to create a new project session, bypassing the system folder picker.

Changes

• Add onDragEnter/Over/Leave/Drop handlers to sidebar root container
• Use preload's webUtils.getPathForFile to obtain folder path (File.path is empty under contextIsolation:true)
• Validate dropped path via /api/files/browse to reject non-directories
• Show drag-over overlay with dashed border and i18n tooltip
• Counter-based dragDepthRef to handle nested dragleave bubbling
• Non-Electron environments silently no-op
• Add i18n keys: dropToCreateProject, dropFileNotFolder, dropCreateFailed

Motivation

Users often have project folders open in Finder and want to quickly start a session. Currently they must use the system folder picker dialog. Drag-and-drop is faster and more intuitive.

Test plan

1. Open CodePilot in Electron
2. Drag a folder from Finder onto the sidebar → should create a new project
3. Drag a single file → should show "not a folder" error toast
4. Drag over sidebar → dashed border overlay appears; drag away → overlay disappears
5. In browser (non-Electron) → drag silently does nothing

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Co-Authored-By: Claude Fable 5 noreply@anthropic.com

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