fix(mobile): responsive modals + landing preview overflow

- PrepareSessionModal: bottom-sheet on mobile, centered on desktop
- IntakeFormModal: bottom-sheet on mobile, responsive padding
- ShareTreeModal: bottom-sheet on mobile, full-width on small screens
- Landing preview: hide URL bar and window controls on mobile (<900px)
  to prevent 189px horizontal overflow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/library/ShareTreeModal.tsx

@@ -112,7 +112,7 @@ export function ShareTreeModal({ tree, isOpen, onClose }: ShareTreeModalProps) {
   if (!isOpen) return null
 
   return (
-    <div className="fixed inset-0 z-50 flex items-center justify-center">
+    <div className="fixed inset-0 z-50 flex items-end justify-center p-0 sm:items-center sm:p-4">
       {/* Backdrop */}
       <div
         className="absolute inset-0 bg-black/80 backdrop-blur-xs"
@@ -120,7 +120,7 @@ export function ShareTreeModal({ tree, isOpen, onClose }: ShareTreeModalProps) {
       />
 
       {/* Modal */}
-      <div className="relative w-full max-w-lg bg-card border border-border rounded-2xl shadow-lg">
+      <div className="relative w-full max-w-full rounded-t-2xl bg-card border border-border shadow-lg sm:max-w-lg sm:rounded-2xl">
         {/* Header */}
         <div className="flex items-center justify-between border-b border-border px-6 py-4">
           <h2 className="text-lg font-semibold text-foreground">Share Tree</h2>

frontend/src/components/procedural/IntakeFormModal.tsx

@@ -212,8 +212,8 @@ export function IntakeFormModal({ isOpen, fields, treeName, onSubmit, onCancel }
   }
 
   return (
-    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/60 backdrop-blur-xs">
+    <div className="fixed inset-0 z-50 flex items-end justify-center p-0 sm:items-center sm:p-4 bg-black/60 backdrop-blur-xs">
-      <div className="mx-4 w-full max-w-lg rounded-2xl border border-border bg-background shadow-xl">
+      <div className="w-full max-w-full rounded-t-2xl border border-border bg-background shadow-xl sm:max-w-lg sm:rounded-2xl">
         {/* Header */}
         <div className="border-b border-border px-6 py-4">
           <h2 className="text-lg font-semibold text-foreground">Project Information</h2>
@@ -224,7 +224,7 @@ export function IntakeFormModal({ isOpen, fields, treeName, onSubmit, onCancel }
 
         {/* Form */}
         <form onSubmit={handleSubmit}>
-          <div className="max-h-[60vh] space-y-4 overflow-y-auto px-6 py-4">
+          <div className="max-h-[70vh] space-y-4 overflow-y-auto px-4 py-4 sm:max-h-[60vh] sm:px-6">
             {Array.from(groups.entries()).map(([groupName, groupFields]) => (
               <div key={groupName}>
                 {groupName && (

frontend/src/components/procedural/PrepareSessionModal.tsx

@@ -88,9 +88,9 @@ export function PrepareSessionModal({
   }
 
   return (
-    <div className="fixed inset-0 z-50 flex items-center justify-center">
+    <div className="fixed inset-0 z-50 flex items-end justify-center p-0 sm:items-center sm:p-4">
       <div className="absolute inset-0 bg-background/60 backdrop-blur-xs" onClick={onClose} />
-      <div className="relative w-full max-w-lg rounded-2xl border border-border bg-card shadow-2xl">
+      <div className="relative w-full max-w-full rounded-t-2xl border border-border bg-card shadow-2xl sm:max-w-lg sm:rounded-2xl">
         {/* Header */}
         <div className="flex items-center justify-between border-b border-border px-5 py-4">
           <div className="flex items-center gap-2">
@@ -106,7 +106,7 @@ export function PrepareSessionModal({
         </div>
 
         {/* Body */}
-        <div className="max-h-[60vh] overflow-y-auto p-5 space-y-5">
+        <div className="max-h-[70vh] overflow-y-auto p-4 space-y-5 sm:max-h-[60vh] sm:p-5">
           {/* Flow name */}
           <div className="rounded-lg bg-accent px-3 py-2">
             <p className="text-xs text-muted-foreground">Flow</p>

frontend/src/styles/landing.css

@@ -1513,6 +1513,18 @@
     display: none;
   }
 
+  .landing-preview-url-bar {
+    display: none;
+  }
+
+  .landing-preview-window-controls {
+    display: none;
+  }
+
+  .landing-preview-titlebar {
+    justify-content: center;
+  }
+
   .landing-tree-branch {
     gap: 1rem;
   }