feat: Add registration mode setting (invite-only vs public)

Backend:
- Added registrationMode and requireInviteCode to settings endpoint
- Values pulled from REGISTRATION_MODE and REQUIRE_INVITE_CODE env vars
- Defaults to 'invite_only' mode (requires invite code)

Frontend:
- Added Registration Mode dropdown to General settings tab
- Options: 'Invite Only' or 'Public Registration'
- Spans full width on desktop layout
- Settings persist with backend save

This allows admins to toggle between invite-only and public registration modes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

maternal-app/maternal-app-backend/src/modules/admin/dashboard/dashboard.service.ts

@@ -356,6 +356,10 @@ export class DashboardService {
       timezone: process.env.TZ || 'UTC',
       language: 'en',
 
+      // Registration Settings
+      registrationMode: process.env.REGISTRATION_MODE || 'invite_only', // 'public' or 'invite_only'
+      requireInviteCode: process.env.REQUIRE_INVITE_CODE === 'true' || true,
+
       // Security Settings
       enforcePasswordPolicy: true,
       minPasswordLength: 8,

parentflow-admin/src/app/settings/page.tsx

@@ -177,7 +177,7 @@ export default function SettingsPage() {
                 <FormControl fullWidth>
                   <InputLabel>Language</InputLabel>
                   <Select
-                    value={settings.language}
+                    value={settings.language || 'en'}
                     label="Language"
                     onChange={(e) => setSettings({ ...settings, language: e.target.value })}
                   >
@@ -188,6 +188,19 @@ export default function SettingsPage() {
                     <MenuItem value="zh">Chinese</MenuItem>
                   </Select>
                 </FormControl>
+
+
+                <FormControl fullWidth sx={{ gridColumn: { xs: '1', md: 'span 2' } }}>
+                  <InputLabel>Registration Mode</InputLabel>
+                  <Select
+                    value={settings.registrationMode || 'invite_only'}
+                    label="Registration Mode"
+                    onChange={(e) => setSettings({ ...settings, registrationMode: e.target.value })}
+                  >
+                    <MenuItem value="invite_only">Invite Only (Requires Invite Code)</MenuItem>
+                    <MenuItem value="public">Public Registration (Open to All)</MenuItem>
+                  </Select>
+                </FormControl>
             </Box>
           </TabPanel>