0.17 phase 4 / PR C — TUI: adaptive layout, scroll, filter

[StanMarek]

Summary

• config edit adapts: 3-pane at width >=100, 2-pane >=60, 1-pane below.
  Preview pane collapsible (p) at medium widths.
• Field list now uses Paragraph::scroll with offset state; PgUp/PgDn/
  Home/End scroll it. Long sections no longer silently clip.
• / opens a filter prompt; key + help substring matched
  case-insensitively. Esc clears.
• Footer shows "Matched N / M fields" while filtered.

Test plan

• cargo test --workspace green (all crates; tui crate alone runs 81 tests including 17 new ones).
• filter_narrows_field_list and scroll_offset_clamped_to_field_count
  cover the new state.
• Headless render smoke via ratatui::backend::TestBackend at 30, 50,
  80, 120 cols, plus the filtered + FilterInput prompt variants —
  every layout branch in render() exercised end-to-end.
• Manual smoke (resize a real terminal): not done by me; flagging
  explicitly per CLAUDE.md ("if you can't test the UI, say so
  explicitly rather than claiming success"). The 7 render smoke unit
  tests replace the no-panic side of the manual smoke; visual
  verification still wanted before merge.

Coordination

• Disjoint from PR A (tui/fields.rs test mod append) and PR B
  (status.rs / main.rs / README.md / SECURITY.md). My only fields.rs
  reference is all_fields() reads from App::new, unchanged.
• Disjoint from phase 5 (gc-suggest providers, gc-pty handler, specs).