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/auth/LoginScreen.tsx

@@ -60,7 +60,7 @@ export default function LoginScreen() {
 
   if (confirmSent) {
     return (
-      <div className="flex h-screen items-center justify-center bg-slate-50">
+      <div className="flex h-dvh items-center justify-center bg-slate-50">
         <div className="w-full max-w-sm rounded-lg border border-slate-200 bg-white p-6 text-center shadow-sm">
           <h1 className="text-sm font-semibold text-slate-800">Check your email</h1>
           <p className="mt-2 text-xs text-slate-500">
@@ -91,7 +91,7 @@ export default function LoginScreen() {
   }
 
   return (
-    <div className="flex h-screen items-center justify-center bg-slate-50">
+    <div className="flex h-dvh items-center justify-center bg-slate-50">
       <form onSubmit={handleSubmit} className="w-full max-w-sm rounded-lg border border-slate-200 bg-white p-6 shadow-sm">
         <h1 className="text-sm font-semibold text-indigo-700">Diagrav</h1>
         <p className="mt-1 text-xs text-slate-500">{mode === 'sign-in' ? 'Sign in' : 'Create an account'}</p>