fix: preserve viewport across MCP-driven file reloads

[trmquang93]

Summary

• When the MCP server mutates the .drawd file externally, the editor reloads via importFlow and applyPayload, which was resetting the viewport (pan/zoom), scope filter, and screen selection — causing a disorienting canvas jump.
• Viewport: Skip setPan/setZoom when source === 'mcp' so the user's current canvas position is preserved.
• Selection: Added preserveSelection option to replaceAll — keeps the selected screen if its ID still exists in the incoming data, clears it otherwise.
• Scope: Preserve scopeRoot during MCP reloads instead of resetting to null.

Test plan

• New useFileActions.test.js (8 tests): verifies viewport/scope/selection preservation for MCP source, and normal viewport application for file opens
• New replaceAll preserveSelection tests in useScreenManager.test.js (3 tests): verifies selection kept when screen still exists, cleared when removed, cleared by default
• npm run lint passes
• npx vite build passes
• npm test — all 850 tests pass (2 pre-existing MCP server failures from missing satori dep)

Closes #8.4