Replace Naive UI form and input primitives

[nighca]

Background

As part of #3013, we scanned the current naive-ui dependency surface in spx-gui.

The largest direct dependency cluster is the form / input layer.

Current dependency points include:

• src/components/ui/form/UIForm.vue
• src/components/ui/form/UIFormItem.vue
• src/components/ui/form/UIFormItemInternal.vue
• src/components/ui/UITextInput.vue
• src/components/ui/UINumberInput.vue
• src/components/ui/UISlider.vue
• src/components/ui/UISwitch.vue
• src/components/ui/checkbox/UICheckbox.vue
• src/components/ui/checkbox/UICheckboxGroup.vue
• src/components/ui/radio/UIRadio.vue
• src/components/ui/radio/UIRadioGroup.vue

Problem

The current form/input abstraction still relies heavily on naive-ui, and in several places it depends on internal behavior instead of stable public contracts:

• UIForm wraps NForm
• UIFormItem wraps NFormItem
• UIFormItemInternal imports formItemInjectionKey from naive-ui/es/_mixins/use-form-item and hijacks internal injection to intercept blur/input behavior
• UITextInput and UINumberInput rely on deep selectors against naive-ui class names such as n-input-*
• UITextInput and UINumberInput also work around autofocus behavior manually
• UISlider is built on NSlider
• checkbox / radio / switch controls are still thin wrappers over naive-ui

This layer is the biggest remaining reason the project still depends on naive-ui.

Goal

Replace the remaining naive-ui form and input primitives with internal components while preserving the current external UI* APIs where reasonable.

Scope

• replace NForm / NFormItem
• remove the dependency on naive-ui internal injection APIs
• replace NInput, NInputNumber, NSlider, NSwitch, NCheckbox, NCheckboxGroup, NRadio, and NRadioGroup
• keep the validation behavior currently expected by the app, including submit handling, validation state display, and IME-safe input behavior
• reduce style coupling to naive-ui class names

Notes

This is likely the largest child task under #3013, and may need to be split again later by control type if execution becomes too broad.

[nighca]

Split this issue into smaller child issues by control family so execution can proceed independently:

• Replace Naive UI form container and validation plumbing #3019 Replace Naive UI form container and validation plumbing
• Replace Naive UI text input primitive #3020 Replace Naive UI text input primitive
• Replace Naive UI number input and slider primitives #3021 Replace Naive UI number input and slider primitives
• Replace Naive UI checkbox, radio, and switch controls #3022 Replace Naive UI checkbox, radio, and switch controls

The split follows implementation boundaries in the current code:

• form orchestration / validation plumbing
• text-like input
• scalar numeric / range input
• boolean / discrete choice controls

This should make it easier to sequence migration work and reduce scope creep inside a single frontend refactor task.