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/App.tsx

@@ -32,7 +32,7 @@ export default function App() {
   }, [session])
 
   if (session === undefined) {
-    return <div className="flex h-screen items-center justify-center text-sm text-slate-400">Loading…</div>
+    return <div className="flex h-dvh items-center justify-center text-sm text-slate-400">Loading…</div>
   }
 
   if (!session) {
@@ -40,7 +40,7 @@ export default function App() {
   }
 
   if (needsUsername === undefined) {
-    return <div className="flex h-screen items-center justify-center text-sm text-slate-400">Loading…</div>
+    return <div className="flex h-dvh items-center justify-center text-sm text-slate-400">Loading…</div>
   }
 
   if (needsUsername) {