Add reusable tb-list widget to webapp widgets

[Copilot]

Adds a generic tb-list Angular widget that renders a Material list with single-item selection and disabled-item support.

New widget: tensorboard/webapp/widgets/list/

• ListComponent (tb-list) — accepts items: ListItem[] and selectedValue, emits selectedValueChange on click; skips emission for disabled items
• ListItem interface — { value: any; label: string; disabled?: boolean }
• ListModule — wraps MatListModule + CommonModule
• Styles — selected highlight, disabled dimming + pointer-events: none, hover state

Supporting infra

• expect_angular_material_list Bazel target added to tensorboard/webapp/angular/BUILD

Usage

<tb-list
  [items]="[{value: 'a', label: 'Option A'}, {value: 'b', label: 'Option B', disabled: true}]"
  [selectedValue]="selected"
  (selectedValueChange)="selected = $event"
></tb-list>

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