refactor(theme): QueryaUiColors and context.colors, the app's own colour type (#1730)

[ZhuchkaTriplesix]

Closes #1730. Part of #1465 (step 3a); the per-folder migration is #1731.

• QueryaUiColors (core/theme/querya_ui_colors.dart, no shadcn import): immutable, lerp, ==, with the fields lib/ reads of shadcn's scheme under the same names: background, foreground, card, popover / popoverForeground, primary / primaryForeground, secondary / secondaryForeground, muted / mutedForeground, accent, destructive / destructiveForeground, border, ring, chart1–chart5, brightness. (The issue named it QueryaColors; that name is the existing palette of constants, so the type is QueryaUiColors.)
• The bridge lives in querya_color_scheme.dart, a file already on the shadcn list: ColorScheme.uiColors (cached per scheme instance) and QueryaUiColors.toShadcn(). Fields shadcn has and the app does not read follow their neighbours, as every Querya theme sets them (cardForeground and accentForeground = foreground, input = border). It goes with refactor(app): replace ShadcnApp with MaterialApp and our theme scope (#1465) #1735.
• QueryaTheme.colors gives the theme's colours as QueryaUiColors.
• context.colors existed and returned shadcn's scheme (29 callers); it now returns QueryaUiColors. It reads the shell theme when there is one, so a caller sees exactly what Theme.of(context).colorScheme gives, mid theme animation included; without a shell (a bare app in a test) it falls back to the nearest QueryaThemeScope, then the default theme of the ambient brightness, instead of asserting.
• Callers that named the type: QueryaDropdown (QueryaThemeColorScheme, the typedef is removed) and the two connection dialogs' card rows.

No visual change: the values are the shadcn scheme's, field by field.

Tests (querya_ui_colors_test.dart): for the dark, light and an imported VS Code theme every field equals the shadcn field, and toShadcn() gives the same scheme back (including the three derived fields); one scheme gives one cached instance; lerp ends and middle; context.colors under the shell, under a bare app, and under a scope without a shell. dart analyze is clean; theme, shared, guards, features, app and e2e suites pass locally.