refactor(agent-activity): show Agent Activity as a centred modal instead of full screen

[Juliusolsson05]

Motivation

The owner finds the full-viewport Agent Activity view (#1170, PR #1105) too much: it covers the whole window, including every pane it is summarizing. It should be an ordinary centred modal built on the shared Dialog primitive (components/ui/dialog.tsx), like Close Old Agents and Close Completed Agents.

Intended behavior

• Agent Activity opens as a centred dialog of bounded size, with the list scrolling inside it.
• Nothing changes except its size and placement: sections, filter, keyboard model, multi-select, bulk close and input ownership all stay the same.

Acceptance criteria

• DialogContent no longer overrides the primitive's positioning to fill the viewport.
• The sticky section headers use the dialog surface colour, not the canvas colour.
• The existing AgentActivityView renderer tests still pass.