feat(sidebar): group objects by type and add Object Browser context menus (#222)

Sidebar tree now groups database objects into Tables, Views, Procedures,
and Functions folders instead of a flat list. Empty groups are hidden
and each group shows its item count.

Object Browser rows now have right-click context menus with actions
like View Data, New Query, View Source, and Drop.

src/types/database.ts

@@ -167,10 +167,16 @@ export type TreeNodeType =
   | "schema"
   | "table"
   | "view"
+  | "procedure"
+  | "function"
   | "group-columns"
   | "group-indexes"
   | "group-fkeys"
   | "group-triggers"
+  | "group-tables"
+  | "group-views"
+  | "group-procedures"
+  | "group-functions"
   | "object-browser"
   | "saved-sql-root"
   | "saved-sql-folder"