Fix UI layout: dynamic editor/results split and scrollable help

[muk2]

Summary

• Fixed autocomplete popup positioning to use the dynamic editor_height_percent instead of a hardcoded Percentage(40), so the popup correctly tracks the editor pane when resized with Ctrl+Shift+Up/Down
• Made the help overlay scrollable (Up/Down/PageUp/PageDown) and adaptive to small terminal sizes, so all keyboard shortcuts are accessible regardless of terminal dimensions
• Added scroll hint to the help overlay footer

What Changed

• src/ui/app.rs: Added help_scroll field to App struct; added scroll handling (Up/Down/PageUp/PageDown) to handle_help_input()
• src/ui/components.rs: Fixed Constraint::Percentage(40) → Constraint::Percentage(app.editor_height_percent) for autocomplete positioning; rewrote draw_help_overlay() to support scrolling and dynamic height

Test Plan

• Resize the editor/results split with Ctrl+Shift+Up/Down, then trigger autocomplete (type 2+ chars or Ctrl+Space) — the popup should appear correctly positioned relative to the editor cursor
• Open the help overlay (?) in a small terminal (e.g., 40 rows) — should show as much content as fits, with scroll hint at bottom
• Scroll the help overlay with Up/Down/PageUp/PageDown — all shortcuts should be reachable
• Press Esc or ? to close help — scroll position resets
• Run cargo test — all 230 tests pass

Closes #61

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