feat: export entire database as SQL file

Add a new "Export Database" feature accessible from the sidebar
right-click menu on database/schema nodes. Generates a .sql file
containing CREATE TABLE DDL, batched INSERT statements, and
view/procedure/function source code.

Backend (database_export.rs):
- Stream-writes SQL to file with configurable batch size
- Supports all SQL database types via existing DDL/INSERT generators
- Progress events with cancellation support
- Graceful error handling (continues on per-table failures)

Frontend (DatabaseExportDialog.vue):
- Connection/database/schema selector
- Checkboxes: structure, data, objects
- Real-time progress with current table and row count
- Cancel button

Wired up through Tauri commands, Web SSE routes, and API layer.

src/composables/useDialogSources.ts

@@ -13,6 +13,7 @@ const showTableImportDialog = ref(false);
 const showStructureEditorDialog = ref(false);
 const showFieldLineageDialog = ref(false);
 const showDatabaseSearchDialog = ref(false);
+const showDatabaseExportDialog = ref(false);
 const showImportLayoutConfirm = ref(false);
 const pendingImportLayout = ref<SidebarLayout | null>(null);
 const showConfigPassphraseDialog = ref(false);
@@ -51,6 +52,9 @@ const lineagePrefillColumn = ref("");
 const databaseSearchPrefillConnectionId = ref("");
 const databaseSearchPrefillDatabase = ref("");
 const databaseSearchPrefillSchema = ref("");
+const databaseExportPrefillConnectionId = ref("");
+const databaseExportPrefillDatabase = ref("");
+const databaseExportPrefillSchema = ref("");
 
 let watchersRegistered = false;
 
@@ -183,6 +187,19 @@ export function useDialogSources() {
         }
       },
     );
+
+    watch(
+      () => connectionStore.databaseExportSource,
+      (v) => {
+        if (v) {
+          databaseExportPrefillConnectionId.value = v.connectionId;
+          databaseExportPrefillDatabase.value = v.database;
+          databaseExportPrefillSchema.value = v.schema ?? "";
+          showDatabaseExportDialog.value = true;
+          connectionStore.databaseExportSource = null;
+        }
+      },
+    );
   } // end watchersRegistered
 
   // Config export/import helpers
@@ -258,6 +275,7 @@ export function useDialogSources() {
     showStructureEditorDialog,
     showFieldLineageDialog,
     showDatabaseSearchDialog,
+    showDatabaseExportDialog,
     showImportLayoutConfirm,
     pendingImportLayout,
     showConfigPassphraseDialog,
@@ -295,6 +313,9 @@ export function useDialogSources() {
     databaseSearchPrefillConnectionId,
     databaseSearchPrefillDatabase,
     databaseSearchPrefillSchema,
+    databaseExportPrefillConnectionId,
+    databaseExportPrefillDatabase,
+    databaseExportPrefillSchema,
     onExportClick,
     onExportConfirm,
     onImportClick,