fix: improve database error message extraction for queries

- frontend: use e.message instead of String(e) in toErrorResult to avoid
  "Error: " prefix for HTTP-mode errors, matching connectionErrorMessage
- frontend: apply same fix to MongoDocBrowser error display
- backend(postgres): use pg_error_to_string across all query execution
  paths (execute, execute_batch, copy_out, copy_in) to extract DbError
  details instead of generic transport-layer errors

Other databases (MySQL, SQL Server, SQLite, DuckDB) already include
server-level error details in their native Error::to_string().

apps/desktop/src/components/mongo/MongoDocBrowser.vue

@@ -167,7 +167,7 @@ async function load() {
     documents.value = result.documents.map(asRecord);
     total.value = result.total;
   } catch (e: unknown) {
-    error.value = String(e);
+    error.value = e instanceof Error ? e.message : String(e);
   } finally {
     loading.value = false;
   }
@@ -354,7 +354,7 @@ async function saveDoc() {
       editJson.value = JSON.stringify(documents.value[selectedIdx.value], null, 2);
     }
   } catch (e: unknown) {
-    error.value = String(e);
+    error.value = e instanceof Error ? e.message : String(e);
   }
 }
 
@@ -371,7 +371,7 @@ async function applyDeleteDoc(idx: number) {
     }
     await load();
   } catch (e: unknown) {
-    error.value = String(e);
+    error.value = e instanceof Error ? e.message : String(e);
   }
 }