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/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;
   }