Add migration plan for SenderPdfImageEditor toolbar

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.

SENDER_PDF_TOOLBAR_PLAN_EN.md

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+# Sender PDF Toolbar Migration Plan
+
+## Goal
+Design and integrate a reusable toolbar component for `SenderPdfImageEditor` that matches the provided visual style and supports:
+- page navigation
+- zoom in/out (including overriding default mouse-based zoom behavior inside editor)
+- PDF download as original bytes (not image)
+- print output
+
+Critical rule: existing signature add/remove/drag-drop behavior must remain fully functional.
+
+## Scope and Files
+Planned touch points:
+1. `EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Components/PdfEditor/SenderPdfImageEditorToolbar.razor` (new)
+2. `EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Components/PdfEditor/SenderPdfImageEditorToolbar.razor.cs` (optional, if code-behind needed)
+3. `EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Components/PdfEditor/SenderPdfImageEditor.razor` (compose toolbar + editor integration)
+4. `EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Pages/SenderEditorPage.razor` (pass PDF bytes/metadata/events to editor)
+5. `EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/envelope-viewer.css` (toolbar visual styles + responsive)
+6. `EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/js/envelope-editor.js` (helper functions: scroll-to-page, wheel handling, download, print if needed)
+
+## Non-Regression Strategy
+- Keep all existing signature event paths unchanged (`EditorClick`, `MoveField`, `RemoveField`).
+- Introduce toolbar as a separate component with explicit parameters and callbacks.
+- Add features incrementally; no behavior switch all at once.
+- After each stage: run manual black-box checks focused on signatures and new feature.
+- If any regression appears: rollback only the stage-local changes.
+
+## Stage Plan
+
+### Stage 1 - Visual Toolbar Skeleton (all buttons disabled)
+- Build standalone `SenderPdfImageEditorToolbar` component.
+- Recreate layout from reference image (three grouped blocks: page nav, zoom, print/download).
+- Render in `SenderPdfImageEditor` above page canvas area.
+- All controls disabled; only visual state and responsive behavior implemented.
+- No functional binding yet.
+
+Acceptance:
+- Toolbar appears and matches structure/spacing/icons closely.
+- Signature add/remove/drag still works exactly as before.
+
+### Stage 2 - Page Navigation
+- Add current page/total page model and callbacks.
+- Implement first/prev/next/last navigation.
+- Implement page number selector UI behavior.
+- Navigate by scrolling/focusing target page container (`data-editor-page`) without changing signature logic.
+
+Acceptance:
+- Navigation controls move to correct page boundaries.
+- Signature interactions continue to work on all pages.
+
+### Stage 3 - Zoom In/Out + Mouse Override
+- Introduce zoom state (e.g., 50%-300%, step-based).
+- Wire `-` / `+` and percent display.
+- Apply zoom in a way that preserves coordinate normalization for signature placement/drag.
+- Override wheel behavior inside editor container (`@onwheel:preventDefault` + JS helper where needed).
+
+Acceptance:
+- Zoom buttons work consistently.
+- Mouse wheel behavior follows custom zoom logic.
+- Signature placement and drag coordinates remain correct after zoom changes.
+
+### Stage 4 - Download Original PDF Bytes
+- Expose download action from toolbar.
+- Download source must be original PDF bytes (same content as uploaded/loaded document).
+- Suggested filename from current editor state.
+
+Acceptance:
+- Downloaded file opens as valid PDF.
+- Not an image export; true PDF bytes.
+
+### Stage 5 - Print Output
+- Add print action from toolbar.
+- Print original PDF content using browser print flow (hidden iframe/object URL approach).
+- Ensure action is isolated and does not mutate editor state.
+
+Acceptance:
+- Print dialog opens with correct document.
+- Existing editor/signature interactions remain intact before and after print.
+
+## Approval Gates
+- Stop after each stage.
+- Share what changed + exact black-box checklist.
+- Wait for explicit approval before moving to next stage.