ui(settings): QueryaSwitch, a nicer toggle for Preferences in the kit's style #1488

Problem

The switches in Preferences (auto-update check, animate theme changes, confirm DML staging, and the other settings rows) are Material Switches with a theme that was never designed for the kit:

• the knob is a large black circle on the cyan track in the dark theme, and an almost invisible track when off; the off state reads as disabled;
• the size does not follow the control scale (sm 28 / md 32 / lg 36) and sits slightly off the label baseline;
• no keyboard focus ring in the kit's style, a ripple from Material instead of the standard hover / pressed states;
• the toggle animates with Material's timing, not QueryaMotion, and ignores Motion Off;
• it is the last Material control in the settings next to QueryaButton, QueryaDropdown and QueryaSearchField, and part of leaving shadcn_flutter / Material look (epic(ui): remove shadcn_flutter from the app #1465, step 4: Switch, Checkbox, Card).

Scope

• A kit widget QueryaSwitch in lib/shared/widgets/, built on Flutter primitives: track and knob from context.workbench (accent on, borderSubtle / muted off, onAccent knob), a track height tied to QueryaControlSize, hover, pressed, disabled and keyboard-only focus ring as in QueryaButton.
• Motion from QueryaMotion (instant under Motion Off and reduced OS motion).
• Semantics: toggled, label, enabled; Space / Enter toggle; the whole settings row stays clickable (PreferencesSwitchRow).
• Use it in PreferencesSwitchRow and the other Switch call sites (mongo_aggregation_workspace.dart), remove the Material path.
• Same for the checkbox rows in a follow-up if it fits (PreferencesCheckboxRow).
• Tests: states and colours in dark and light themes, toggling by pointer and keyboard, semantics, one height next to a QueryaButton of the same size; add it to the UI kit gallery (dev(ui): UI kit gallery: every control, variant, size and state in every theme #1336) and to docs/ui-kit.md.

Acceptance

• On and off are clearly different in the dark and the light theme and in imported VS Code themes; disabled is distinct from off.
• The switch is the height of its control size and centred on its label.
• Keyboard focus shows the kit's ring; a mouse click does not.
• No material.Switch in lib/; a guard keeps it that way.