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-tauri/src/commands/database_export.rs

@@ -0,0 +1,53 @@
+use std::sync::Arc;
+use tauri::{AppHandle, Emitter, State};
+
+use crate::commands::connection::AppState;
+
+pub use dbx_core::database_export::{DatabaseExportRequest, ExportProgress, ExportStatus};
+
+fn emit_progress(app: &AppHandle, progress: ExportProgress) {
+    let _ = app.emit("database-export-progress", progress);
+}
+
+#[tauri::command]
+pub async fn export_database_sql(
+    app: AppHandle,
+    state: State<'_, Arc<AppState>>,
+    request: DatabaseExportRequest,
+) -> Result<(), String> {
+    let state = state.inner().clone();
+    let export_id = request.export_id.clone();
+
+    tokio::spawn(async move {
+        let result = dbx_core::database_export::export_database_sql_core(&state, &request, |progress| {
+            emit_progress(&app, progress)
+        })
+        .await;
+
+        if let Err(e) = result {
+            emit_progress(
+                &app,
+                ExportProgress {
+                    export_id: export_id.clone(),
+                    current_object: String::new(),
+                    object_index: 0,
+                    total_objects: 0,
+                    rows_exported: 0,
+                    total_rows: None,
+                    status: ExportStatus::Error,
+                    error: Some(e),
+                },
+            );
+        }
+
+        dbx_core::database_export::clear_export_cancelled(&export_id).await;
+    });
+
+    Ok(())
+}
+
+#[tauri::command]
+pub async fn cancel_database_export(export_id: String) -> Result<(), String> {
+    dbx_core::database_export::set_export_cancelled(&export_id).await;
+    Ok(())
+}