Introduce a detailed migration plan to design and integrate a reusable toolbar component for the `SenderPdfImageEditor` in the `EnvelopeGenerator.Server` project.
The toolbar will support page navigation, zoom in/out, PDF download, and print functionality while preserving existing signature add/remove/drag-drop behavior.
The plan includes:
- Creation of a new `SenderPdfImageEditorToolbar` component.
- Updates to `.razor`, `.css`, and `.js` files.
- A non-regression strategy with incremental feature additions and manual black-box testing after each stage.
The migration is divided into five stages:
1. Visual toolbar skeleton.
2. Page navigation.
3. Zoom in/out with mouse override.
4. Download original PDF bytes.
5. Print output.
Approval gates are defined to ensure functionality is implemented correctly and regressions are avoided.
