refactor: Modularize database page into components and composables

- Extract types to types/database.ts
- Create composables: useDatabaseApi, useDataTable, useQueryExecutor
- Create components: DatabaseSidebar, DataTable, FilterBar, SchemaInfo,
  QueryEditor, QueryColumnsBar, DatabaseStats, TablePagination
- Add horizontal scroll to DataTable with sticky checkbox column
- Configure @ path alias in vite and tsconfig
- Reduce DatabasePage.vue from 1548 to 314 lines

frontend/src/composables/database/useQueryExecutor.ts

@@ -0,0 +1,53 @@
+import { ref } from 'vue'
+
+const API_BASE = 'http://localhost:4101/api/database'
+
+export function useQueryExecutor() {
+  const queryText = ref('')
+  const queryResult = ref<any[] | null>(null)
+  const queryError = ref<string | null>(null)
+  const queryLoading = ref(false)
+
+  async function executeQuery() {
+    if (!queryText.value.trim()) return
+
+    queryLoading.value = true
+    queryError.value = null
+    queryResult.value = null
+
+    try {
+      const res = await fetch(`${API_BASE}/query`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ query: queryText.value })
+      })
+
+      const result = await res.json()
+
+      if (!res.ok) {
+        queryError.value = result.error || 'Query failed'
+      } else {
+        queryResult.value = result.rows
+      }
+    } catch (e: any) {
+      queryError.value = e.message
+    } finally {
+      queryLoading.value = false
+    }
+  }
+
+  function reset() {
+    queryText.value = ''
+    queryResult.value = null
+    queryError.value = null
+  }
+
+  return {
+    queryText,
+    queryResult,
+    queryError,
+    queryLoading,
+    executeQuery,
+    reset
+  }
+}