Add mobile view-only support

Below md width, the app now forces view-only (same canEdit mechanism as a
shared view-only diagram) rather than trying to support touch editing:
palette and inspector panels hide, editing controls disable, and the top
bar collapses to just Diagrams + Account. A bottom tab bar swaps between a
full-screen canvas and a full-screen Bill of Materials, since there's no
room for both side by side. Canvas gets a few mobile-specific trims too:
MiniMap and the interactivity lock button hide whenever canEdit is false
(the lock button doesn't actually grant editing either way — nodesDraggable/
nodesConnectable already override it — so leaving it visible would just be
misleading), and h-screen is replaced with h-dvh throughout so the layout
doesn't get clipped by a mobile browser's collapsing address bar.

Also fixes quick-add's anti-stack offset (24px — far smaller than a device
node) to actually tile devices apart instead of leaving them nearly
overlapping, and logs a deferred "Auto-arrange" layout feature in
organized-ideas.md, surfaced by mobile visitors having no way to fix an
overlapping diagram themselves.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017DUU6CnxECCDeqDNYJgr5x

src/components/layout/MobileTabBar.tsx

@@ -0,0 +1,28 @@
+export type MobileTab = 'diagram' | 'bom'
+
+/** Bottom nav for the mobile view-only layout (AppShell) — stands in for
+ *  the palette + RightPanel that only fit at `md` and up. Just two views:
+ *  there's no editing to switch an Inspector tab for on mobile (see
+ *  useCanEdit), so "Diagram" and "Bill of Materials" are the whole set. */
+export default function MobileTabBar({ tab, onChange }: { tab: MobileTab; onChange: (tab: MobileTab) => void }) {
+  return (
+    <nav className="flex h-12 shrink-0 border-t border-slate-200 bg-white md:hidden">
+      <button
+        onClick={() => onChange('diagram')}
+        className={`flex-1 text-xs font-semibold uppercase tracking-wide ${
+          tab === 'diagram' ? 'text-indigo-700' : 'text-slate-400'
+        }`}
+      >
+        Diagram
+      </button>
+      <button
+        onClick={() => onChange('bom')}
+        className={`flex-1 text-xs font-semibold uppercase tracking-wide ${
+          tab === 'bom' ? 'text-indigo-700' : 'text-slate-400'
+        }`}
+      >
+        Bill of Materials
+      </button>
+    </nav>
+  )
+}