refactor: standardize page title typography across all workspaces

All page-level headers now use fontSize:1.125rem, fontWeight:700,
color:#0f172a, container py:2.5, borderBottom:#e8e7e4 — matching
the standard established in PageHeader. Removes variant="h5"/h6"
inconsistencies across Supply, Demand, and shared components.

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

src/components/supply/ReminderHeader.tsx

@@ -14,17 +14,16 @@ export function ReminderHeader() {
     <Box
       sx={{
         bgcolor: 'white',
-        borderBottom: '1px solid',
-        borderColor: 'divider',
+        borderBottom: '1px solid #e8e7e4',
         px: 3,
-        py: 2,
+        py: 2.5,
         display: 'flex',
         alignItems: 'flex-start',
         justifyContent: 'space-between',
       }}
     >
       <Box>
-        <Typography variant="h5" sx={{ fontWeight: 700, color: '#1e293b' }}>
+        <Typography sx={{ fontWeight: 700, fontSize: '1.125rem', lineHeight: 1.3, color: '#0f172a' }}>
           Reminder Manager
         </Typography>
         <Typography variant="body2" color="text.secondary">