Add image modal view for image files on tickets

Done.
New apps/web/src/lib/components/tickets/ImageModal.svelte is a native <dialog> lightbox: image fits the viewport (max height calc(100vh - 8rem)), dark blurred backdrop, close X top-right, body scroll locked while open. The image itself is the only non-dismissive zone, clicking the caption, the dark area, the close button, or pressing escape all close. Non-image attachments still open in a new tab.

Used in TicketAttachments.svelte: image tiles render as a <button> with cursor: zoom-in that sets a previewID state; non-image tiles keep the open-in-new-tab anchor.

Tests

• New ImageModal.svelte.test.ts covers render content, close-button click, null-attachment guard, caption-area click closes, and image click does not close

# /apps/api tests
Test Files  17 passed (17)
Tests  122 passed (122)

Code: https://github.com/jmmd2000/issues/commit/18fa69d8d44155dc2983935c3b1a754123b0ad9d
Tests: https://github.com/jmmd2000/issues/commit/e78f68e8dfc8381b4ed168f98acabad9c1b0af52