refactor(viewcube): shrink cube 30%, compact controls, centre UCS picker

Reduce the ViewCube 30% (VIEWCUBE_PX 120→84) and tighten the nav region
(NAV_INSET_F 2.2→2.0) so the home / roll / nudge controls hug the cube in the
corner instead of floating in a large dead area, with sizes scaled to keep them
from overlapping. Bump the home and roll buttons back up slightly, shift the
UCS picker down below the region, and give it a fixed width so it centres
exactly under the cube centre.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/pipeline/viewcube.rs

@@ -15,14 +15,15 @@ use iced::wgpu;
 use iced::{Rectangle, Size};
 
 // ── ViewCube layout ───────────────────────────────────────────────────────
-pub const VIEWCUBE_PX: u32 = 120;
+pub const VIEWCUBE_PX: u32 = 84; // 30% smaller cube (was 120)
 pub const VIEWCUBE_SCALE: f32 = 0.36;
 pub const VIEWCUBE_DRAW_PX: f32 = VIEWCUBE_PX as f32 * VIEWCUBE_SCALE * 2.0;
 pub const VIEWCUBE_PAD: f32 = 12.0;
 /// The cube centre is inset from the screen corner by this multiple of the
 /// cube half-size, leaving room for the compass ring and the nav controls
-/// (home / roll / nudge) drawn around it.
-pub const NAV_INSET_F: f32 = 2.2;
+/// (home / roll / nudge) drawn around it. Tighter than before so the controls
+/// hug the cube in the corner instead of floating in a large dead region.
+pub const NAV_INSET_F: f32 = 2.0;
 /// Side of the whole nav widget (cube + compass ring + controls) in pixels.
 pub const VIEWCUBE_REGION_PX: f32 = VIEWCUBE_DRAW_PX * NAV_INSET_F;
 /// Z height of the compass ring + cardinals in cube-local space. The cube