style(buttons): standardise button sizes across all components

Define two button tiers and apply consistently:
- sm (inline pairs): padding 0.5rem 1rem, font-size 0.875rem, radius 6px
- md (standalone CTAs): padding 0.625rem 1.25rem, font-size 0.9rem, radius 8px

RankingsView: viewButton was 0.25rem/0.8rem/4px vs addButton 0.5rem/0.875rem/8px
— biggest mismatch, both now sm with same radius.
SchoolCard: horizontal padding 0.75rem → 1rem, font-size 0.8125rem → 0.875rem.
SchoolRow: padding 0.4375rem → 0.5rem to match sm exactly.
SchoolDetailView: font-size 0.8125rem → 0.875rem.
ComparisonView/Modal: addButton and shareButton aligned to md spec.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/components/SchoolCard.module.css

@@ -148,8 +148,8 @@
 .btnSecondary,
 .btnPrimary {
   flex: 1;
-  padding: 0.5rem 0.75rem;
-  font-size: 0.8125rem;
+  padding: 0.5rem 1rem;
+  font-size: 0.875rem;
   font-weight: 600;
   border-radius: 6px;
   border: none;
@@ -190,8 +190,8 @@
 
 .btnRemove {
   flex: 1;
-  padding: 0.5rem 0.75rem;
-  font-size: 0.8125rem;
+  padding: 0.5rem 1rem;
+  font-size: 0.875rem;
   font-weight: 600;
   border-radius: 6px;
   cursor: pointer;