fix: set canvas-layout class on initial server render

When loading the rspace canvas directly (not via tab navigation),
#app lacked the canvas-layout class. Without it, #canvas collapses
to 0 height and pointer events for panning miss the canvas element.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/shell.ts

@@ -91,7 +91,7 @@ export function renderShell(opts: ShellOptions): string {
   <div class="rstack-tab-row" data-theme="${theme}">
     <rstack-tab-bar space="${escapeAttr(spaceSlug)}" active="" view-mode="flat"></rstack-tab-bar>
   </div>
-  <main id="app">
+  <main id="app"${moduleId === "rspace" ? ' class="canvas-layout"' : ''}>
     ${body}
   </main>