fix: search input focus not working when modal opens (#463)

The search modal was using `visibility: hidden` to hide the backdrop,
which prevented the input from receiving focus. This caused:
- No text cursor on desktop when pressing Cmd+K
- No keyboard popup on mobile when tapping search icon

Changed to use only `opacity: 0` (already present) for hiding.
Elements with `opacity: 0` remain focusable, allowing the synchronous
focus() call in search.open() to work correctly - critical for mobile
keyboards which only open when focus is in the same call stack as the
user gesture.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[e-mail]>

frontend/src/lib/components/SearchModal.svelte

···
 		align-items: flex-start;
 		justify-content: center;
 		padding-top: 15vh;
-		/* hidden by default */
+		/* hidden by default - use opacity only (not visibility) so input remains focusable for mobile keyboard */
 		opacity: 0;
-		visibility: hidden;
 		pointer-events: none;
-		transition: opacity 0.15s, visibility 0.15s;
+		transition: opacity 0.15s;
 	}
 
 	.search-backdrop.open {
 		opacity: 1;
-		visibility: visible;
 		pointer-events: auto;
 	}