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/Modal.module.css

@@ -123,11 +123,13 @@
   .modal {
     width: 100%;
     max-width: 100%;
-    /* Bottom sheet: open at a stable, comfortable height so the empty
-       "start typing" state isn't a tiny stub and the sheet doesn't jump as
-       results load (the results list scrolls within). */
-    min-height: 55vh;
-    max-height: 95vh;
+    /* Bottom sheet sized against the overlay (which tracks the visual
+       viewport), NOT vh: when the keyboard is open the overlay is short, so
+       max-height:100% keeps the whole sheet — input and results — above the
+       keyboard. min-height gives a comfortable default without a tiny stub,
+       but is capped at 100% so it never exceeds the visible area. */
+    min-height: min(55vh, 100%);
+    max-height: 100%;
     border-bottom-left-radius: 0;
     border-bottom-right-radius: 0;
     animation: slideUp 0.3s ease;