New component: timeline

[fabianhueske]

New mt-timeline component: chronological event sequence.

• Ordered items (marker + connector), ascending/descending
• Item: marker (icon/dot), title, optional timestamp/description/actor — slot-driven
• Semantic marker variants (neutral/success/warning/critical) + current item
• Vertical (optional horizontal)
• Optional grouping/dividers (e.g. by day)
• Loading + empty states; long-list handling