Fix #434: action menu opens below trigger button via placement=Below

[JDonaghy]

Summary

The editor tab bar … action menu opened at the row of the button itself (overlapping the tab row) instead of below it. After PR #427 migrated the action menu from gtk4::PopoverMenu to engine-drawn quadraui::ContextMenu, the click coords were anchored as-is and the adapter never set a placement — quadraui defaulted to AnchorPoint (top-left == click). TUI had been masking the same bug with a row + 1 hack at the call site.

Take option 1 from the issue: thread the trigger element's height through the engine and use ContextMenuPlacement::Below.

• ContextMenuState gains trigger_height: f32 (line_height units; supports sub-cell rows like GTK's 1.6× tab row). 0.0 = right-click flow → AnchorPoint; non-zero = dropdown-style → Below.
• open_editor_action_menu(group_id, x, y, trigger_height) — new arg. The four right-click open_* paths default to 0.0.
• ContextMenuPanel + render builder propagate the field.
• context_menu_panel_to_quadraui_context_menu sets placement based on trigger_height.
• Both backends switch from .layout() to .layout_at() with an anchor Rect carrying the trigger height in their native unit.
• GTK click handler passes tab_row_height_px / line_height (≈1.6) so the menu sits flush against the button with zero gap.
• TUI call sites drop the row + 1 hack and pass (row, 1.0).

Auto-flip to Above when there's no room below is handled by quadraui's layout_at — vimcode gets it for free.

Test plan

• cargo build clean
• cargo test --no-default-features --lib — 1965 passing, 0 failing
• cargo clippy -- -D warnings clean
• cargo fmt --check clean
• Smoke (GTK): action button menu opens flush below the button, no gap; right-click flows still anchor at click point
• Smoke (TUI): confirmed flush

Closes #434

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