Add drag-and-snap tiling layout canvas to Plot Builder

Replace layout radio buttons (Stacked/Side-by-Side/Grid) with a visual
LayoutCanvas widget. Pane tiles snap to grid cells on drag-drop; dropping
on an occupied cell swaps positions. Ghost highlight shows snap target
during drag.

- PaneSpec gains row/col fields (None = unassigned; backward compat preserved)
- LayoutConfig gains "free" mode; strip_chart reads explicit positions
- PaneBlock loses ▲▼ move buttons; reordering is canvas-only
- QSS entries added for layoutTile, layoutTileDragging, layoutCanvasBar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 40 insertions, 0 deletions

diff --git a/ui/style_dark.qss b/ui/style_dark.qss
index 2c24d1c..e562365 100644
--- a/ui/style_dark.qss
+++ b/ui/style_dark.qss
@@ -563,3 +563,43 @@ QFrame#controlWidgetWrapper {
     background: transparent;
     border: none;
 }
+
+/* ── Layout canvas ───────────────────────────────────────────────── */
+QWidget#layoutCanvasBar {
+    background-color: #0f1521;
+    border-bottom: 1px solid #1c2540;
+}
+QLabel#layoutCanvasLabel {
+    font-family: "IBM Plex Mono", monospace;
+    font-size: 10px;
+    font-weight: 700;
+    letter-spacing: 2px;
+    color: #4338ca;
+    padding: 0 2px;
+}
+QFrame#layoutCanvasWidget {
+    background-color: #0b0e13;
+    border: 1px solid #2a3558;
+    border-radius: 4px;
+}
+QFrame#layoutTile {
+    background-color: #1c2540;
+    border: 1px solid #2a3558;
+    border-radius: 5px;
+}
+QFrame#layoutTile:hover {
+    border-color: #4338ca;
+    background-color: #212c52;
+}
+QFrame#layoutTileDragging {
+    background-color: #1e3a5f;
+    border: 2px solid #3b82f6;
+    border-radius: 5px;
+}
+QLabel#layoutTileLabel {
+    font-family: "IBM Plex Mono", monospace;
+    font-size: 11px;
+    font-weight: 700;
+    color: #8b9dc3;
+    background: transparent;
+}