Add maintenance mode to data analysis pages

Restore all data analysis pages but display them in maintenance mode.
Users can see these pages in the navigation and access them, but they
show a maintenance message instead of trying to fetch data.

Changes:
- Create MaintenanceMode component for reusable maintenance UI
- Restore all deleted pages: explorer, metadatos, rawExplorer, panorama,
  comparativa-cosechas, informe-ingresos
- Replace complex data-fetching logic with MaintenanceMode component
- Add "Mantenimiento" badges to navigation menu items
- Pages show clear maintenance message with technical details
- Users can still navigate to these pages without errors

This approach:
- Keeps the UI structure intact
- Shows users what features exist
- Provides clear communication about maintenance status
- Prevents confusion about missing features
- Easy to re-enable when data sources are reconnected

nuxt4-app/app/pages/explorer.vue

@@ -0,0 +1,15 @@
+<template>
+  <MaintenanceMode
+    title="Explorador de Datos"
+    description="El explorador de datos está temporalmente deshabilitado mientras realizamos mejoras en el sistema de consultas."
+    icon="i-lucide-table"
+    technical-info="La funcionalidad de exploración de datos requiere reconexión con la fuente de datos. Actualmente en proceso de migración a nueva arquitectura."
+  />
+</template>
+
+<script setup lang="ts">
+definePageMeta({
+  layout: 'dashboard',
+  title: 'Explorador de datos'
+})
+</script>