Add responsive grid layout for mobile/tablet support

Switch from fixed 48-column grid to responsive breakpoints:
- lg (>=1200px): 48 columns, free positioning (unchanged desktop behavior)
- md (>=900px): 24 columns, vertical compaction
- sm (>=600px): 12 columns, vertical compaction
- xs (<600px): 6 columns, vertical compaction

Widgets automatically reflow and stack on smaller screens instead of
being squished. Layout changes are only persisted from the desktop
breakpoint. Drag/resize editing is desktop-only.

Also adds mobile CSS refinements for topbar, tabs, and workspace padding.

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

next-env.d.ts

@@ -0,0 +1,6 @@
+/// <reference types="next" />
+/// <reference types="next/image-types/global" />
+import "./.next/dev/types/routes.d.ts";
+
+// NOTE: This file should not be edited
+// see https://nextjs.org/docs/app/api-reference/config/typescript for more information.