feat(ui): unified viewport control chip + per-tile grid rendering

Move grid and grid-snap toggles off the status bar (and its customization
menu) into the top-left viewport chip, now a single container holding the
horizontal/vertical split buttons, render-mode picker, and grid/snap
toggles, sized to the render-mode picker. Add grid + magnet (snap) icons.

Render the grid per model tile from each tile's own grid_on, in its own
bounds and camera, instead of a single grid in the active tile. This stops
the grid flickering on/off as the cursor crosses panes or leaves the
viewport, and lets each pane show its own grid independently.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/icons/ui/grid.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#000000" stroke-width="1.8" stroke-linejoin="round"><rect x="4" y="4" width="16" height="16" rx="1.5"/><path d="M4 9.3 H20 M4 14.7 H20 M9.3 4 V20 M14.7 4 V20" stroke-width="1.4"/></svg>

assets/icons/ui/snap.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#000000" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round"><path d="M6 5 V11 a6 6 0 0 0 12 0 V5"/><path d="M6 5 H10 M14 5 H18" stroke-width="2.6"/></svg>