fix: re-add ..Default::default() to ContextMenuItem literals

[JDonaghy]

Summary

• Commit `8a53307` (merged via PR feat: cache bottom-panel geometry; resolve_bottom_panel_zone (#418) #424) removed the `..Default::default()` lines on four `quadraui::ContextMenuItem` literals in `src/render.rs` that `a516f76` had added to keep develop compiling against the newer quadraui field shape (`checked`, `key_equivalent`, `submenu` added in quadraui `daed293`).
• Without these lines, develop fails to build with E0063 — confirmed locally on a fresh checkout.
• This PR re-applies the same 4-line fix. No behavior change; the doc on `ContextMenuItem` explicitly recommends struct-update syntax for forward compat.

Test plan

• `cargo build` — clean
• `cargo clippy -- -D warnings` — no warnings
• `cargo test --no-default-features --lib` — 1963 passed / 0 failed

Discovered while smoke-testing PR #427 (action menu migration). #427 will be rebased onto develop once this lands.

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