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/RankingsView.module.css

@@ -271,7 +271,7 @@
   font-size: 0.875rem;
   font-weight: 600;
   border: none;
-  border-radius: 8px;
+  border-radius: 6px;
   cursor: pointer;
   transition: all 0.2s ease;
   background: var(--accent-coral, #e07256);
@@ -372,11 +372,12 @@
 
 .viewButton {
   display: inline-block;
-  padding: 0.25rem 0.625rem;
-  font-size: 0.8rem;
+  padding: 0.5rem 1rem;
+  font-size: 0.875rem;
+  font-weight: 500;
   color: var(--text-secondary);
   border: 1px solid var(--border-color, #e0ddd8);
-  border-radius: var(--radius-sm, 4px);
+  border-radius: 6px;
   text-decoration: none;
   margin-right: 0.375rem;
   transition: all var(--transition, 0.2s ease);