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/stores/connectionStore.ts

@@ -91,6 +91,11 @@ export const useConnectionStore = defineStore("connection", () => {
     database: string;
     schema?: string;
   } | null>(null);
+  const databaseExportSource = ref<{
+    connectionId: string;
+    database: string;
+    schema?: string;
+  } | null>(null);
   const sidebarLayout = ref<SidebarLayout>(emptyLayout());
   let layoutPersistTimer: ReturnType<typeof setTimeout> | null = null;
 
@@ -1400,6 +1405,7 @@ export const useConnectionStore = defineStore("connection", () => {
     structureEditorSource,
     fieldLineageSource,
     databaseSearchSource,
+    databaseExportSource,
     createConnectionGroup(name: string) {
       const result = createGroupOp(sidebarLayout.value, name);
       updateLayoutAndRebuild(result.layout);