TASK-017 TUI: In-panel search/filter with /

Description

When a panel is focused, pressing / opens an inline filter input at the bottom of the panel. Typing filters the panel's items in real time (case-insensitive substring match). Esc clears the filter and closes the input. Enter locks the filter (input hides, filtered items remain visible with a [filter: "…"] badge in the border title).

Applicable panels: News, Feeds, HN, Sports, Calendar.

Implementation:

• Add a filter: str = "" attribute to DashPanel or the item-layer widget
• Pressing / focuses an Input widget docked at the bottom of the panel
• on_input_changed re-renders the content filtered to matching items
• Esc clears filter and dismisses the input
• Enter keeps the filter active but hides the input; border title shows […]

Depends on: TASK-005 (focus), TASK-007 (item navigation)

Acceptance Criteria

• #1 / when panel is focused opens inline filter input
• #2 Items filter in real time as user types
• #3 Esc clears filter and hides input
• #4 Active filter shown in border title
• #5 Works in News, Feeds, HN, Sports, Calendar panels