feat(ui): add mobile preview device framing and design QA tools

vibn-frontend/app/api/chat/route.ts

@@ -217,6 +217,12 @@ Each project has a persistent \`vibn-dev\` container. Edit files via \`fs_*\` an
 
 **Testing Auth & Protected Routes:** Do NOT attempt to verify signup flows or authenticated routes by making HTTP requests (e.g. \`curl\` or \`http_fetch\`) to the dev server yourself. The app is protected by NextAuth or similar session cookies which you do not have. Just write the code, start the dev server via \`dev_server_start\`, and provide the user the clickable \`previewUrl\` so they can test it themselves in their browser. If you hit a redirect/401, do NOT assume the server is broken and loop on restarting it.
 
+**Design Critique / Visual QA Tool:**
+- \`request_visual_qa { targetPath }\` runs a fast background AI agent to critique a UI file (like \`page.tsx\`, \`layout.tsx\`, or \`.css\`) against a strict 5-dimensional design rubric (Layout, Spacing, Contrast, Hierarchy, Responsiveness).
+- You MUST call this tool whenever your turn involves creating or heavily modifying visual User Interface code before you return the \`previewUrl\` to the user.
+- If the tool returns a failure with actionable issues (e.g., "missing mobile padding" or "using hardcoded colors instead of CSS variables"), you MUST use \`fs_edit\` to fix those specific issues before ending your turn.
+- Do NOT use this tool if you only modified backend code, SQL, config files, or non-visual logic.
+
 **Rules:**
 - Stay under \`/workspace\`. \`fs_*\` enforce this; use \`shell_exec\` deliberately for system paths.
 - Dev container has no route to internal Vibn services (vibn-postgres, etc.) by design.