feat: add signal bind methods to flow components

[Artur-]

Add bindX(Signal/WritableSignal) methods across multiple components for signal bindings that keep component state synchronized with signal values while attached.

One-way bindings (Signal → component):

• CheckboxGroup.bindRequired
• ComboBoxBase.bindRequired
• ProgressBar.bindIndeterminate
• BigDecimalField.bindLocale
• Details.bindOpened (WritableSignal, two-way)
• Markdown.bindContent + Markdown(Signal) constructor
• Grid.bindColumns, Column.bindHeader/bindFooter, bindEmptyStateText
• Tooltip.bindText

Two-way bindings (WritableSignal ↔ component):

• AppLayout.bindDrawerOpened
• Checkbox.bindIndeterminate
• Crud.bindDirty
• MenuItemBase.bindChecked
• SideNav.bindExpanded
• SplitLayout.bindSplitterPosition
• Tabs.bindSelectedIndex
• Map.bindCenter, Map.bindZoom (via moveend event write-back)

Includes unit tests for all bind methods.

[vaadin-bot]

TC Format Checker Report - 13:00 - 18 - Feb

There are 2 files with format errors

• To see a complete report of formatting issues, download the differences file

• To fix the build, please run mvn spotless:apply in your branch and commit the changes.

• Optionally you might add the following line in your .git/hooks/pre-commit file:

  mvn spotless:apply
  

Here is the list of files with format issues in your PR:

vaadin-app-layout-flow-parent/vaadin-app-layout-flow/pom.xml
vaadin-crud-flow-parent/vaadin-crud-flow/pom.xml

[yuriy-fix]

@Artur-, are the draft signal PRs going to be closed? Are those for the testing purposes?