refactor: swap Button ghost/outline semantics, clean up ImageLightbox thumbnail

ghost now means transparent bg (no fill); outline keeps cream bg with subtle border.
Remove magnify icon overlay from ImageLightbox thumbnail — hover cursor-zoom-in is sufficient.
Close button updated to variant="outline" for cream-on-blue contrast in the dialog.

src/shared/styles/theme.css

@@ -439,4 +439,5 @@
 /* Lightbox dialog backdrop */
 dialog.lightbox::backdrop {
   background-color: rgba(4, 28, 243, 0.25);
+  backdrop-filter: blur(4px);
 }