TASK-030 TUI: Preserve panel scroll position across data refreshes

Description

Every time refresh_data() runs, all DashPanel content is replaced via set_content(), which resets the scroll position to the top. If a user has scrolled halfway through a news feed, a background refresh jumps them back to the top.

Implementation:

• Before set_content(), save panel.scroll_y (or panel.scroll_offset)
• After set_content() (which replaces the Static), restore the saved scroll position via panel.scroll_to(y=saved_y, animate=False)
• Only restore if the new content height ≥ saved scroll position (avoid restoring to a position that no longer exists after content shrinks)
• Apply to all DashPanel instances

Acceptance Criteria

• #1 Scroll position is maintained across automatic background refreshes
• #2 Manual r refresh also preserves scroll
• #3 Scroll resets to top when the panel's source kind changes or content is significantly shorter
• #4 No visible scroll jump during updates