fix: scope Tab search-box binding to the canvas container

Address accessibility (WCAG 2.1.1) feedback: a global bare Tab binding
would hijack sequential focus navigation across all UI controls. Scope it
to graph-canvas-container (same pattern as Delete, Fit View, etc.) so Tab
only opens the search box when focus is on the canvas, leaving normal Tab
navigation intact everywhere else.

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

src/platform/keybindings/defaults.ts

@@ -36,6 +36,13 @@ export const CORE_KEYBINDINGS: Keybinding[] = [
     },
     commandId: 'Workspace.ToggleSidebarTab.workflows'
   },
+  {
+    combo: {
+      key: 'Tab'
+    },
+    commandId: 'Workspace.SearchBox.Toggle',
+    targetElementId: 'graph-canvas-container'
+  },
   {
     combo: {
       key: 'n'