refactor: GTK action menu engine-drawn; drop dead right-click bodies (#395)

[JDonaghy]

Summary

• Action menu migrated to engine-drawn path. The editor `…` action menu was the last popover-on-editor-DA still building a native `gtk4::PopoverMenu`. The engine-drawn ctx-menu path (paint at `draw.rs::draw_context_menu_popup`, click dispatch + hover via cached `ContextMenuLayout` at `gtk/mod.rs:6022`, keyboard via `engine::handle_context_menu_key`) was already fully wired by earlier work (feat: GTK tab/editor context menus use engine-drawn rendering #137, ContextMenu: motion handlers should use primitive's hit_test, not hand-rolled row math #210/fix: context menu click/hover uses cached ContextMenuLayout hit_test (#210) #425). Two changes hook it up: `click.rs` derives `(col, row)` from the click's pixel coords when opening the menu (was hardcoded `(0, 0)`) so the anchor lands under the `…` button, and the `show_action_menu_popover` call in `mod.rs:7113` is dropped — engine state + existing render + click dispatch take over.
• 3 unused functions removed (~497 lines): `show_action_menu_popover` (now unused), `handle_tab_right_click` and `handle_editor_right_click` (both already replaced by `Msg::TabRightClick` / `Msg::EditorRightClick` in earlier PRs but left behind under `#[allow(dead_code)]`).
• Explorer right-click NOT migrated here — scope split to GTK explorer context menu: migrate to engine-drawn ctx menu (cross-DA coord handling) #426. Engine-drawn ctx menu paints on the editor DA, but explorer right-clicks happen on the separate explorer DA with its own coord system; the migration needs cross-DA coordinate handling that warrants its own PR. `show_explorer_context_menu` + its 3 helpers (`build_gio_menu_from_engine_items`, `swap_ctx_popover`, `menu_row_count`) + the `active_ctx_popover` field stay in place for now.

Refs #395 (does not close — explorer follow-up is #426).

Files

• `src/gtk/click.rs` — `+3 / -1`: pass real `(col, row)` to `open_editor_action_menu`.
• `src/gtk/mod.rs` — `+4 / -507`: drop `show_action_menu_popover` call + 3 unused functions.
• `SUMMARIES/gtk_mod.md` — line-count refresh.

Test plan

Smoke checklist added as a PR comment so the validating agent has it inline.

🤖 Generated with Claude Code

[JDonaghy]

Smoke test checklist (for the desktop agent picking this up)

1. Quality gate

cargo fmt
cargo clippy -- -D warnings
cargo test --no-default-features --lib
cargo build

All four must be green before merging. The clippy run is the critical one — this PR removed ~500 lines and changed the action-menu code path; clippy will catch any orphan imports, unused fields, or dead code that I might have missed.

2. Action menu smoke (the actual migrated piece)

This is the only behavior change in the PR. Everything else is dead-code removal.

• Open a file in vimcode (GTK). Click the … action menu button on the right side of the tab bar.
• Menu appears under the … button, not at top-left of the editor. (Before the click.rs anchor fix, it would have rendered at (0, 0).)
• Click each item — verify the action fires:
  • Close All → all tabs close
  • Close Others / Close Saved / Close to the Right / Close to the Left
  • Word Wrap: On / Off → toggles :set wrap
  • Change Language → opens language picker
  • Reveal in Files → opens file manager at the file's directory
• Click outside the menu → menu dismisses.
• Press Esc → menu dismisses.
• Press j / k → selection moves up/down inside the menu.
• Press Enter on a selected item → action fires.

3. Regression checks — context menus that were already engine-drawn

These weren't touched by this PR but exercise the same engine-drawn path; verify nothing drifted:

• Tab right-click: right-click on a tab → menu appears under the cursor. Click items (Close, Close Others, Close to the Right, Copy Path, Split). Each fires.
• Editor right-click: right-click inside the editor → context menu (Cut/Copy/Paste/Go to Definition/etc.). Items fire.

4. NOT changed in this PR — explorer right-click

• Right-click a file or folder in the explorer sidebar → native GTK PopoverMenu still appears (as before). Menu items (New File, Rename, Delete, etc.) still fire normally.

If the explorer menu looks visually different or items don't fire, that's a regression. The explorer migration is deferred to #426.

5. Branch ready-to-merge state

If everything above passes, this PR is ready for merge to develop. The CLAUDE.md workflow step 5 then says: when merged, run gh issue close <N> and unassign — but this PR only refs #395 (doesn't close it), since the explorer follow-up at #426 is what completes the original issue's scope. Keep #395 open until #426 also lands.