feat(ui5-timeline): introduce header-bar slot

[hinzzx]

Overview

Introducing a header-bar slot to ui5-timeline with a new ui5-timeline-header-bar component that provides search, filter, and sort controls. The Timeline itself remains a pure renderer — filtering and sorting are the application's responsibility via events.

What We Did

• Added ui5-timeline-header-bar component with search input, sort toggle, and filter dialog
• Added ui5-timeline-filter-option component for declarative filter choices
• Added TimelineSortOrder enum (Ascending / Descending)
• Added headerBar slot to Timeline and forwarded search, filter, sort events
• Updated Timeline.css to use flexbox layout for the header-bar + content split
• Updated existing Cypress tests to use attribute selectors ([ui5-timeline-item]) and realClick()
• Added new Cypress tests covering search, filter, sort, and header-bar rendering
• Added three website samples: WithHeaderBar, WithSearch, WithFilter
• Registered new components in ReactPlayground and Monaco types
• Added i18n keys for all accessible names, tooltips, and dialog labels

What This Adds

• Search — text input fires search event on every keystroke; app filters items in the DOM
• Filter — dialog with multi-select list fires filter event with selected option texts; empty selection = show all
• Sort — toggle button fires sort event with Ascending or Descending; sorting is always active
• Accessibility — all controls have accessibleName/tooltips via i18n;

Quick Demo

[ui5-webcomponents-bot]

🧹 Preview deployment cleaned up: https://pr-13155--ui5-webcomponents.netlify.app

[github-actions]

Sample sync reminder

HTML sample changed but React sample (sample.tsx) not updated:

• fiori/Timeline/WithFilter
• fiori/Timeline/WithHeaderBar
• fiori/Timeline/WithSearch

Please keep both samples in sync, or ignore if the change does not apply to both.