add text zoom, fix context panel drag, widen default panels

Ctrl+Plus/Minus/0 text zoom (5% steps, 60%–200%) via MediaQuery
textScaler.  App-wide clideLineHeight (1.25) applied in DefaultTextStyle.

Context panel drag resize was inverted — dragging left shrank the
panel instead of growing it.  Negate delta for right-anchored slots.

Default sidebar widened from 240px to 400px, context panel from 280px
to 420px (both at their previous maximums).

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

lib/kernel/src/panels/drag_resize.dart

@@ -72,9 +72,10 @@ class _DragResizeHandleState extends State<DragResizeHandle> {
     final start = _dragStartSize;
     final startPt = _dragStartPointer;
     if (start == null || startPt == null) return;
-    final delta = widget.axis == Axis.horizontal
+    final rawDelta = widget.axis == Axis.horizontal
         ? e.position.dx - startPt.dx
         : e.position.dy - startPt.dy;
+    final delta = widget.slot == Slots.contextPanel ? -rawDelta : rawDelta;
     widget.arrangement.setSize(widget.slot, start + delta);
   }