Bump gpui-kit to 0.7

[pavi2410]

Summary

• Bump `gpui-kit` 0.6 → 0.7.3 (GPUI snapshot `gpui-pre` 0.3.4 → 0.3.9).
• Open the main, About, Settings and pop-out windows with `gpui_kit::open_window`; `Root` is now created and overlay-hosted by Kit.
• Remove `chrome/overlays.rs` (manual dialog/sheet/notification layers no longer exist).
• Drop the `PaneRef::Tiles` arm in `dock_utils.rs` (tiles canvas removed upstream).

Test plan

• `cargo check`, `cargo clippy --workspace --all-targets` (no errors), `cargo test --workspace`
• Manually verify dialogs/sheets/notifications in main window and pop-outs
• Check result grid and Markdown rendering (table widths, TextView typography changed)
• Check `tree-sitter-sql` highlighting

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