Replace hand-rolled dropdowns and dialog with accessible primitives

Dropdown and SortDropdown are divs with click handlers: no keyboard support, no roles, no aria-expanded, unusable with a screen reader and awkward with a keyboard. Dialog has no focus trap, no role="dialog", and Escape doesn't close it. This is solved-problem territory, not worth maintaining by hand.

• Adopt Radix (or Base UI) primitives for dropdown menu, select and dialog, styled with the existing Tailwind look
• Replace Dropdown, SortDropdown and Dialog usages
• Add labels to the unlabelled inputs (admin password, search box, review textarea)
• aria-labels for icon-only buttons (colour picker add/remove, bookmark)