[ZhuchkaTriplesix]

Closes #1729. Part of #1465.

• gap: ^3.0.1 is a direct dependency now (the version shadcn already pulled in, so nothing changes at runtime). The barrel exports package:gap/gap.dart and hides Gap, MaxGap, SliverGap from its shadcn re-export.
• Every unprefixed import 'package:shadcn_flutter/...' hides Gap, and a file that uses Gap without the barrel imports package:gap/gap.dart (including result_grid_view.dart, whose part files use it). No file takes Gap from shadcn any more.
• Seven files no longer import shadcn at all: for them it was only Gap and Flutter types (vertical_split_pane, erd_table_card, erd_search_panel, update_available_badge, query_editor_tab, grid_column_sizer, grid_selection_manager). 59 -> 52 files import the package.
• shadcn_imports_ratchet_test: lists the 52 files that still import shadcn; a new file may not start, and a file that drops the import must leave the list. A second test checks that no unprefixed shadcn import brings Gap.

Found on the way, for #1735: shadcn has its own ThemeMode (ThemeController uses it), so querya_core_commands.dart cannot take ThemeMode from Material until the shell moves.

dart analyze is clean; guards and the workspace, ERD, Mongo, main-screen, shared, layout and updater suites pass locally (1096 tests).