TASK-019 TUI: Inline summary overlay for news/feed items

Description

Briefs items have a summary field. News and feeds items may carry summaries too depending on source config. When a user selects an item with a summary and presses Space, show a popup overlay with the full summary text.

Implementation:

• Create a SummaryScreen(ModalScreen) that displays the item title + full summary in a scrollable box
• Bind Space on a selected item to push SummaryScreen if item.summary is non-empty
• Overlay is 80% width, auto height, dismissible with Esc or Space
• If no summary available, show a brief "no summary" toast instead of a blank overlay

Depends on: TASK-007 (keyboard-navigable items)

Acceptance Criteria

• #1 Space on a selected item with a summary opens the overlay
• #2 Overlay shows full title and summary text
• #3 Overlay is scrollable for long summaries
• #4 Esc dismisses the overlay
• #5 Items without summaries show a toast, not an empty overlay