Add unified workspace file previews backed by enhanced citations

## Summary
Add a more intentional file preview experience for workspace files, preferably through an explicit Preview action that opens a popup modal using the same patterns as enhanced citations. The goal is to make files easier to inspect from workspace surfaces while also broadening and hardening enhanced citation previews.

## User Value
Users need to quickly confirm that the right file was uploaded, indexed, shared, or cited without downloading it or starting a chat. A reusable preview pattern also reduces the gap between workspace file management and chat citation inspection.

## Proposed Behavior
- Add a visible Preview action for workspace files where a preview is supported. Prefer this over hover as the primary interaction so the feature works well on desktop, keyboard, and touch devices.
- Reuse or extract the enhanced citation modal behavior for source file previews where practical.
- Support existing enhanced citation preview types, including images, PDFs, audio/video, tabular files, and Visio files.
- For unsupported files, show a clear fallback such as metadata, indexed text, or download/open actions rather than failing silently.
- Keep chat enhanced citation previews working as they do today while sharing reusable preview code where possible.

## Acceptance Criteria
- [ ] Workspace document rows/cards expose a Preview action for supported file types.
- [ ] Preview opens an accessible modal with file name, type-aware preview content, loading/error states, and fallback actions.
- [ ] Preview behavior works for personal, group, and public workspace contexts where the user has access.
- [ ] Enhanced citation modals and workspace file previews use a consistent supported-file matrix and fallback behavior.
- [ ] Backend preview endpoints enforce existing workspace authorization and do not expose raw storage URLs, secrets, or cross-workspace files.
- [ ] Existing chat citation preview behavior remains covered and does not regress.
- [ ] Add focused functional/UI coverage for supported preview types and unauthorized access/fallback cases.

## Notes
- Related but broader than #972, which focuses on reusing the image popup for chat and workspace images.
- Likely implementation areas include `application/single_app/static/js/workspace/workspace-documents.js`, `application/single_app/static/js/chat/chat-enhanced-citations.js`, and `application/single_app/route_enhanced_citations.py`.
- Open design question: whether to add hover previews later for lightweight metadata/snippets after the explicit Preview action is in place.