Dialog primitive: per-backend native-look styling option

[JDonaghy]

Idea

The `quadraui::Dialog` primitive is functional but visually generic — it looks the same on every backend (a centered box with title, body, buttons). Some users would prefer dialogs that match their platform's native look:

• GTK: rounded corners, GNOME / Adwaita button styling, header bar styling, OK/Cancel button order convention (Cancel on left, primary on right per GNOME HIG).
• Win-GUI (future): Win11 dialog styling — rounded corners, Mica/Acrylic background, Yes/No button order convention.
• macOS (future): NSAlert-like sheet styling — possibly even using AppKit if available behind a feature flag.

Approach options

1. Severity-tinted theming. `DialogSeverity` (already in the primitive) drives an icon + accent. Cheap, doesn't change layout. Could ship soon.
2. Per-backend stylesheet. Each backend's `draw_dialog` reads from a `DialogStyle` table that the theme owns. Lets users override colours / corner radius per platform without touching the primitive.
3. Native widget escape hatch. A new `Backend::show_native_dialog(...)` trait method that bypasses the primitive entirely on backends with a strong native dialog facility (e.g. GTK4 `MessageDialog`, Win32 `MessageBoxW`, NSAlert). Loses cross-backend layout consistency, gains true native feel.
4. Adwaita styling on GTK. Specific to GTK — render with rounded corners + Adwaita-like spacing without consulting native widgets. Lowest-friction first step.

Option (4) is the most pragmatic first pass — small change to `quadraui_gtk::draw_dialog`, no primitive contract change.

Surfaced by

Smoke testing of #205 slice 5 (`7768a25`). User comment: "some way to style these dialogs to make them look more native would be a great feature for later."

Priority

Low / nice-to-have. Functional dialogs ship today; this is polish.

Files

• `src/gtk/quadraui_gtk.rs::draw_dialog` — first place to introduce GTK-specific styling
• `quadraui/src/primitives/dialog.rs::DialogSeverity` — already exists, currently unused
• (Future) `src/win_gui/quadraui_win.rs::draw_dialog` when Win-GUI gets a Dialog consumer

[JDonaghy]

Superseded in direction by #727: option 3 ("native widget escape hatch") is the path being taken, via quadraui#666's PlatformServices::show_message_dialog + gtk4::AlertDialog. Options 1/2/4 (severity tinting, per-backend stylesheet, Adwaita styling of the in-canvas box) still apply to the dialogs that cannot go native — anything carrying a DialogTable or DialogInput, plus every TUI dialog. Retitle to that residue, or close, once #727 lands.