fix: isolate undo history for unsaved documents

[Amandot]

Description

Fixes the issue where Cmd+Z could unexpectedly clear content in newly created unsaved documents.

Root Cause

MonacoEditor was rendered without a path prop, causing different tabs to reuse the same Monaco text model and Undo/Redo history.

When switching between tabs, content synchronization could create synthetic edits in the shared Undo stack. As a result, pressing Cmd+Z could undo beyond the user's actual edits and unexpectedly clear or restore content from another document.

Fix

• Give each editor tab a stable Monaco model identity using activeTab.id.
• Pass the stable ID through EditorPane to Monaco as the path.
• Keep the Monaco model identity unchanged when an unsaved document is saved.
• Isolate Undo/Redo history between tabs.
• Add Ctrl+Shift+Z support for Redo on non-Mac platforms.

Tests

Added regression tests covering:

• Unsaved document Undo
• Multiple Undo operations
• Large content deletion and Undo
• Edit → Undo → Save
• Saved document Undo
• Multiple tab/document isolation
• Redo
• Original root-cause reproduction

Verification

The following checks pass:

• npm test
• npm run lint
• npx tsc --noEmit
• npm run build

Fixes #247