ui(settings): own colour picker and slider for the theme editor and Preferences; Card/Switch leftovers (#1465) #1734

Part of #1465 (step 4c).

Problem

The theme editor and Preferences still use shadcn controls with no kit counterpart: ColorPicker and ColorDerivative (theme_color_picker_dialog.dart, theme_editor_dialog.dart), Slider / SliderValue (preferences_controls.dart, interface scale). Smaller leftovers found by the same probe (dev, 2026-10-18): Card (5, extensions and mongodb), shadcn Switch (1, mongodb), Colors / ThemeMode taken from the shadcn barrel (Material's own work).

Change

• A colour picker of our own for the theme editor: hue / saturation-value area, alpha, hex field, the palette swatches it shows today; same result type (a Color).
• QueryaSlider (or a styled Material Slider) for the interface scale; QueryaSwitch from ui(settings): QueryaSwitch, a nicer toggle for Preferences in the kit's style #1488 covers the switches.
• Card → the kit's card surface; Colors / ThemeMode imported from Material.

Acceptance

• No ColorPicker, ColorDerivative, Slider, SliderValue, Card, Switch from shadcn in lib/.
• Theme editor: pick, edit hex, alpha; tests for the picker's value round trip.