Draw the board as the canvas draws boxes, and drag cards live

The user found the board's look off from the canvas's: a block's C was
gray around a white card, borders ignored the node's style, a folded
column said nothing, and dragging felt wrong.

Columns and cards now go through the canvas's line layer, with the same
rules as Scene::add_borders: the node's border style and color, thick in
the accent color for the cursor, thick in the visual color for the
selection. A block's bracket and foot are drawn in its card's own line
and color, so the C is one shape and its joint comes out as ├, ┣ or ╠.
Tags and badges sit in the bottom border through a footer() shared with
the canvas renderer. Columns show their tags and badge too, so a folded
column says +N, and a column's title can be typed in its top border
(it wasn't shown as typed before).

A drag now moves the card live, as on the canvas (the user's pick), all
one undo step: by the row under the mouse, before a card on its top
half, after it or into its block on its bottom half, out at a foot,
first on a column's title, last below the cards. Columns keep their
widths while dragging, or a column that narrowed would slide the one
under the mouse away and the card would chase it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/view/canvas/lines.rs

@@ -74,13 +74,7 @@ impl LineLayer {
         paint: Paint,
         shown: impl Fn(ScreenPos) -> bool,
     ) {
-        let stroke = match border {
-            Border::Rounded | Border::Square => Stroke::Solid,
-            Border::Double => Stroke::Double,
-            Border::Thick => Stroke::Thick,
-            Border::Dashed => Stroke::Dashed,
-            Border::Hidden => return,
-        };
+        let Some(stroke) = border.stroke() else { return };
         if rect.width < 2 || rect.height < 2 {
             return;
         }
@@ -95,10 +89,8 @@ impl LineLayer {
             if shown(at) {
                 self.add_arm(at, one, stroke, paint);
                 self.add_arm(at, other, stroke, paint);
-                if border == Border::Rounded
-                    && let Some(cell) = self.cell_mut(at)
-                {
-                    cell.rounded = true;
+                if border == Border::Rounded {
+                    self.round(at);
                 }
             }
         }
@@ -174,6 +166,14 @@ impl LineLayer {
         }
     }
 
+    /// Draws the light corner on a cell rounded, as a rounded box's corners
+    /// are.
+    pub fn round(&mut self, at: ScreenPos) {
+        if let Some(cell) = self.cell_mut(at) {
+            cell.rounded = true;
+        }
+    }
+
     /// Every cell a line runs through, with its character and color.
     pub fn cells(&self, glyphs: Glyphs) -> impl Iterator<Item = (ScreenPos, char, Color)> + '_ {
         self.cells.iter().enumerate().filter(|(_, cell)| !cell.arms.is_empty()).map(move |(index, cell)| {