fix(rithmomachia): render guide as docked in preview panel

Fix the guide in the preview panel to render as docked (not as another
floating modal):
- Set docked={true} when guide is in panel (both preview and committed dock)
- Guide now uses relative positioning, no shadow, fills panel properly
- Only show undock button when truly docked (not during preview)
- Preview guide now visually matches the final docked appearance

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/arcade-games/rithmomachia/components/RithmomachiaGame.tsx

@@ -189,8 +189,8 @@ export function RithmomachiaGame() {
                     <PlayingGuideModal
                       isOpen={true}
                       onClose={() => setIsGuideOpen(false)}
-                      docked={guideDocked} // Only truly docked if guideDocked is true
-                      onUndock={handleUndock}
+                      docked={true} // Always render as docked when in panel
+                      onUndock={guideDocked ? handleUndock : undefined} // Only show undock button when truly docked
                       onDockPreview={handleDockPreview}
                     />
                   </Panel>
@@ -226,8 +226,8 @@ export function RithmomachiaGame() {
                     <PlayingGuideModal
                       isOpen={true}
                       onClose={() => setIsGuideOpen(false)}
-                      docked={guideDocked} // Only truly docked if guideDocked is true
-                      onUndock={handleUndock}
+                      docked={true} // Always render as docked when in panel
+                      onUndock={guideDocked ? handleUndock : undefined} // Only show undock button when truly docked
                       onDockPreview={handleDockPreview}
                     />
                   </Panel>