🤖 fix: Windows open-in-editor and use deep links for built-in editors

[ibetitsmike]

Fixes malformed "Open in Editor" links on Windows and refactors Electron to use deep links for built-in editors.

Problem

• Windows paths were being incorrectly formatted in deep links (vscode://fileC:\... instead of vscode://file/C:/...)
• Electron and Browser apps had divergent code paths for opening editors

Solution

Deep Link Normalization

• normalizeLocalPathForEditorDeepLink() converts \ → / and ensures leading / for Windows paths

Unified Editor Launching

• Built-in editors (VS Code, Cursor, Zed): Always use deep links via window.open() in both Electron and Browser
• Custom editors: Backend spawns the configured command (Electron only)

Backend Simplification

• EditorService now only handles custom editor commands
• Removed built-in editor spawning logic from backend

Cleanup

• Removed openInEditorDeepLinkFallback.ts (obsolete with new approach)

Files Changed

• src/browser/utils/editorDeepLinks.ts - Added path normalization
• src/browser/utils/openInEditor.ts - Refactored to use deep links for built-in editors
• src/node/services/editorService.ts - Stripped to custom-only
• Tests updated accordingly

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