PDF Viewer: Add dedicated PDF view mode

[lattln]

eSheet currently supports modes such as build, code, and preview, but there is no dedicated view for rendering or reviewing PDF output. A PDF view is needed so users can inspect the generated PDF experience directly inside the eSheet workflow without relying on a separate external step.

This would make it easier to validate how form content, generated documents, or PDF-based outputs appear after configuration. It also gives builders and reviewers a clear place to test PDF rendering behavior separately from form preview or schema/code inspection.

Problem / Motivation:

The current view modes do not clearly support PDF-focused validation. Preview is useful for interacting with the form, and code/build modes are useful for configuration, but PDF output has different requirements around layout, pagination, rendering, and document review.

Without a dedicated PDF view, PDF-related work is harder to test, debug, and communicate during development or QA.

Acceptance Criteria:

• A new PDF View mode is available alongside the existing build, code, and preview views
• Users can switch into PDF view from the main eSheet interface
• PDF view renders the relevant PDF output or PDF preview area
• PDF view is visually separated from normal form preview behavior
• Empty, loading, and error states are handled clearly
• Existing build, code, and preview modes continue to work as expected
• PDF view does not break existing form editing or schema behavior

Notes / Constraints:

• PDF view should be treated as its own workflow, not as a replacement for form preview
• Initial support can focus on rendering/previewing PDF output before adding advanced PDF actions
• Consider future support for download, print, zoom, pagination, or generated document validation if needed

[redmine-bridge-mieweb]

Redmine Ticket: #158668 created from GitHub Issue