feat: add user settings save and reading plans with progress tracking

User Settings:
- Add /api/user/settings endpoint for persisting theme and fontSize preferences
- Update settings page with working save functionality
- Add validation and localized error messages

Reading Plans:
- Add database schema with ReadingPlan, UserReadingPlan, and UserReadingProgress models
- Create CRUD API endpoints for reading plans and progress tracking
- Build UI for browsing available plans and managing user enrollments
- Implement progress tracking with daily reading schedule
- Add streak calculation and statistics display
- Create seed data with 5 predefined plans (Bible in 1 year, 90 days, NT 30 days, Psalms 30, Gospels 30)
- Add navigation link with internationalization support

Technical:
- Update to MUI v7 Grid API (using size prop instead of xs/sm/md and removing item prop)
- Fix Next.js 15 dynamic route params (await params pattern)
- Add translations for readingPlans in all languages (en, ro, es, it)

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

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

app/[locale]/settings/page.tsx

@@ -126,11 +126,37 @@ export default function SettingsPage() {
   }
 
   const handleSave = async () => {
+    const token = localStorage.getItem('authToken')
+    if (!token) {
+      setMessage(t('settingsError'))
+      return
+    }
+
     try {
-      // TODO: Implement settings update API
-      await new Promise(resolve => setTimeout(resolve, 1000)) // Placeholder
-      setMessage(t('settingsSaved'))
+      const response = await fetch(`/api/user/settings?locale=${locale}`, {
+        method: 'PUT',
+        headers: {
+          'Content-Type': 'application/json',
+          'Authorization': `Bearer ${token}`
+        },
+        body: JSON.stringify({
+          theme: settings.theme,
+          fontSize: settings.fontSize,
+          notifications: settings.notifications,
+          emailUpdates: settings.emailUpdates,
+          language: settings.language
+        })
+      })
+
+      const data = await response.json()
+
+      if (response.ok && data.success) {
+        setMessage(t('settingsSaved'))
+      } else {
+        setMessage(data.error || t('settingsError'))
+      }
     } catch (error) {
+      console.error('Error saving settings:', error)
       setMessage(t('settingsError'))
     }
   }