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>

src/ui/statusbar.rs

@@ -32,7 +32,6 @@ impl StatusBar {
         ortho_mode: bool,
         polar_mode: bool,
         polar_increment_deg: f32,
-        show_grid: bool,
         dyn_input: bool,
         otrack: bool,
         layouts: Vec<String>,
@@ -94,7 +93,6 @@ impl StatusBar {
 
         // ── Right side ────────────────────────────────────────────────────
         let osnap_active = snapper.is_active();
-        let snap_grid_on = snapper.is_on(crate::snap::SnapType::Grid) && snapper.snap_enabled;
 
         let vp_label = if viewport_count > 0 {
             format!("{} VP", viewport_count)
@@ -126,18 +124,6 @@ impl StatusBar {
                 "Cursor coordinates (X, Y, Z)",
             ));
         }
-        if vis(StatusPill::Snap) {
-            right_status = right_status.push(tip(
-                toggle_pill("SNAP", snap_grid_on, Message::ToggleGridSnap),
-                "Snap to Grid\nF9",
-            ));
-        }
-        if vis(StatusPill::Grid) {
-            right_status = right_status.push(tip(
-                toggle_pill("GRID", show_grid, Message::ToggleGrid),
-                "Show Grid\nF7",
-            ));
-        }
         if vis(StatusPill::Ortho) {
             right_status = right_status.push(tip(
                 toggle_pill("ORTHO", ortho_mode, Message::ToggleOrtho),