Add fullscreen image overlay for voting images

[IvoLeist]

Motivation

• Provide a way to view voting images fullscreen to inspect details without leaving the voting workflow.
• Support quick access via a small toggle button, double-click on the image, and Escape or overlay controls to close the fullscreen view.

Description

• Added image-overlay.js to the JS includes in votingUI so the overlay behaviour is loaded (panzoom.min.js, init-panzoom.js, hotkeys.js, image-overlay.js).
• Injected a small overlay toggle button, overlay container, and overlay image into the voting image UI in output$voting_image_div with namespaced IDs (voting_image_overlay, voting_image_overlay_image) and ARIA attributes.
• Implemented overlay styling and layout in inst/shiny-app/www/voting-styles.css including .image-overlay, .image-overlay-toggle, .image-overlay-close, and body.overlay-open to lock scrolling while open.
• Implemented inst/shiny-app/www/js/image-overlay.js to open the overlay (syncing the displayed image src), close it via the close button, backdrop click, Escape key, and via double-click on the image.

Testing

• Performed a local static preview test by writing tmp_overlay_preview.html, serving it with python -m http.server 8001, and running a Playwright script that opened the page, clicked the overlay toggle, waited, and saved a screenshot to artifacts/image-overlay.png, which completed successfully.
• Verified the overlay markup renders in renderUI and the CSS/JS files are present under inst/shiny-app/www during the test run.

---

Codex Task

[IvoLeist]

decided to use the Github Copilot implementation since this it is working now.
Never tested this suggestion by Codex