fix(compare): keep the add-school sheet above the on-screen keyboard

The bottom-sheet overlay was position:fixed inset:0, sized to the LAYOUT
viewport, so when the mobile keyboard opened it kept full height: the
sheet (anchored to the overlay's bottom) and the lower half of the dim
backdrop sat behind the keyboard, hiding the search input and results.

Track window.visualViewport (height + offsetTop) while the modal is open
and pin the overlay to the visible region, so backdrop and sheet stay
above the keyboard. Size the sheet against the overlay (max-height:100%,
min-height:min(55vh,100%)) instead of vh, and drop the results list's own
inner scroll on mobile so the sheet is a single scroll area.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/components/SchoolSearchModal.module.css

@@ -155,6 +155,15 @@
     padding: 1rem;
   }
 
+  /* One scroll container on mobile: the modal content itself scrolls, so the
+     results list must not add its own inner scroll (double scrollbars, and
+     the input would be trapped above a short 400px window when the keyboard
+     shrinks the sheet). */
+  .results {
+    max-height: none;
+    overflow: visible;
+  }
+
   /* Compact stacked card: name + meta, then a full-width action so the tap
      target is obvious and the card doesn't waste vertical space. */
   .resultItem {