refactor(settings, workspace): colours through context.colors (#1731)

[ZhuchkaTriplesix]

Part of #1731 (folders features/settings and features/workspace, ~150 reads). Epic #1465.

Mechanical, no visual change:

• Theme.of(context).colorScheme (one-line and wrapped over several lines) and theme.colorScheme → context.colors;
• the ColorScheme type of parameters and fields → QueryaUiColors (the grid cell, row and header renderers, Preferences panes, the theme picker); the colorScheme field of the grid renderers is colors;
• ThemePreviewCard reads the previewed theme's colours from QueryaTheme.colors.
• Theme.of(context) stays where it gives something that is not a colour (brightness, radiusSm in the destructive-query and DML dialogs and the grid tooltip): the guard forbids colorScheme and ColorScheme, not Theme.of, until the shell moves (refactor(app): replace ShadcnApp with MaterialApp and our theme scope (#1465) #1735).

theme_picker_button.dart no longer imports shadcn (ratchet list: 37 → 36).

Guard colors_through_context_test.dart now covers connections, mongodb, redis, settings and workspace. It caught one wrapped Theme.of(context)\n.colorScheme in results_tab.dart that my search had missed. Two more of that shape are left in onboarding and extensions, for their folders.

dart analyze is clean; guards, settings, workspace, main-screen (goldens included) and e2e suites pass locally (800).