feat: expand FormsScreen with comprehensive form element examples

This commit transforms the `FormsScreen` from a simple placeholder into a detailed showcase of available form components, including state management and code examples for each.

### Key Changes:

*   **Widget Refactor:**
    *   Converted `FormsScreen` from a `StatelessWidget` to a `StatefulWidget` to manage local state for checkboxes, radio buttons, switches, and sliders.

*   **New Form Element Examples:**
    *   **Text & Email Inputs:** Added specialized `FormField` examples for standard text and email validation.
    *   **Date Picker:** Added a `ControlledDatePicker` example with `NonNullValidator`.
    *   **Text Area:** Added a resizable `TextArea` example with configurable initial dimensions.
    *   **Dropdown Menu:** Integrated a `DropdownMenu` example using `withPopover`, demonstrating labels, dividers, and sub-menus.
    *   **Checkboxes:** Added examples for both simple and TriState (unchecked/indeterminate/checked) checkboxes.
    *   **Radio Selection:** Included examples for standard `RadioGroup` lists and `RadioCard` layouts for richer UI options.
    *   **Toggle Switch:** Added a `Switch` example with leading and trailing labels.
    *   **Sliders:** Added both single-value `Slider` and `RangeSlider` examples, including step divisions and value displays.

*   **Layout Adjustments:**
    *   Reduced the maximum width of the content container from `1000` to `752` for better readability.
    *   Standardized spacing between component cards using `Gap(20)`.
    *   Improved code snippet accuracy in `CodeCard` descriptors to match actual implementation logic.

Author: vmeng-b