[Repo Assist] feat(ui): make help overlay scrollable

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🤖 This PR was created by Repo Assist, an automated AI assistant.

What

Makes the ? help overlay scrollable so all keybindings are accessible
on any terminal size.

Before: The help text (36 lines) was rendered as a static
Paragraph with no scroll support. On a typical 24-row terminal the
centered_rect(60, 70, ...) overlay is only ~17 rows of content — roughly
half the keybindings were silently clipped below the bottom border.

After: ↑ ↓ PgUp PgDn Home End (and j/k vim aliases) scroll
the help overlay. A scrollbar appears on the right edge when the content
overflows. The title shows ↑↓ to scroll as a discoverability hint.
Closing the overlay (? or Esc) resets the scroll position to the top.

Why

This is a real usability bug: users on compact terminals (e.g. 24-row
VS Code integrated terminal) could never see the Mouse section or the
General section at the bottom of the help. The fix uses only existing
Ratatui primitives — no new dependencies.

Changes

File	Change
src/app.rs	help_scroll: u16 + help_max_scroll: u16 fields; initialised to 0
src/handler.rs	New handle_help_input() function; called from handle_events when show_help is true
src/ui.rs	draw_help_overlay now takes &mut App; uses .scroll((app.help_scroll, 0)); renders a Scrollbar when content overflows; title updated

Keybindings inside the overlay

Key	Action
↑ / k	Scroll up one line
↓ / j	Scroll down one line
PgUp	Scroll up 10 lines
PgDn	Scroll down 10 lines
Home	Jump to top
End	Jump to bottom
? / Esc	Close overlay (scroll resets to top)

Test Status

cargo check --all-targets  → clean
cargo fmt -- --check       → clean
cargo clippy -- -D warnings → clean
cargo test                 → 238 passed (was 231) — 7 new tests:
  help_scroll_starts_at_zero
  help_scroll_down_increments_within_max
  help_scroll_down_clamped_at_max
  help_scroll_up_saturates_at_zero
  help_scroll_home_resets_to_zero
  help_scroll_end_jumps_to_max
  help_close_resets_scroll

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