feat(viewport): reorder model controls, add close-vport button

Reorder the model-tile control chip to grid, grid snap, render mode,
vertical split, horizontal split. Add a close button (shown only with
more than one model tile) that merges the active pane into its
longest-contact neighbour.

Divider drags now clamp each side to the minimum tile size, so dragging
a boundary to the screen edge no longer collapses a pane — closing is
done with the button instead.

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

src/app/mod.rs

@@ -1078,6 +1078,9 @@ pub enum Message {
     /// Split the active Model viewport in two. `true` → horizontal divider
     /// (top / bottom); `false` → vertical divider (left / right).
     SplitModelViewport(bool),
+    /// Close the active Model viewport, merging it into a neighbour.
+    /// Only meaningful when more than one model tile exists.
+    CloseModelViewport,
     /// Recall previous command in history (↑ arrow key).
     CommandHistoryPrev,
     /// Recall next command in history (↓ arrow key).