feat: entity headers and explore pages layout

[HelderMendes]

Headers and page layouts for all major entity types, and completes the explore section structure.

New entity headers

• DatasetHeaderNew — dataset detail header with key metadata and actions
• TaskHeader — task detail header
• FlowHeader — flow detail header
• RunHeader — run detail header
• MeasureHeader — measure detail header

Explore layout

• (explore)/layout.tsx — shared layout providing consistent navigation and structure across all explore pages

Entity detail pages (updated to use new headers)

• Datasets, Tasks, Flows, Runs, Measures, Collections, Benchmarks

New pages

• collections/create — create a new collection/study
• datasets/upload — dataset upload page
• tasks/create — create a new task
• runs/compare — side-by-side run comparison