feat: Design tab, live style editing, route-aware artboards, canvas onboarding

Design tab (4-tab inspector: Design / Props / Diff / Graph)
- Click any component in a live artboard to inspect computed CSS
- Typography, Layout, Visual sections auto-populated from getComputedStyle
- Every property is inline-editable; changes apply live via PATCH_ELEMENT_STYLE
- Zero source-code modifications — purely in-browser style overrides

Protocol additions (packages/renderer)
- REQUEST_ELEMENT_STYLES / ELEMENT_STYLES round-trip for CSS inspection
- PATCH_ELEMENT_STYLE for live style patching

Live SDK (packages/live-sdk)
- respondWithStyles(): reads 30 curated computed CSS properties
- patchElementStyle(): applies inline style override to fiber's DOM element

Canvas store
- selectedComponentStyles: current element's CSS map
- styleEditEvent mailbox: Design tab → LiveArtboard style dispatch (Zustand v5 compatible)

Route-aware artboards
- Each artboard now has an optional route (e.g. /dashboard)
- buildSrc(base, route) helper in Artboard.tsx
- Route field editable in Inspector Props tab, persisted to metadata_jsonb

On-canvas onboarding
- Empty canvas now shows 3-step guide: press A → CLI command → paste URL
- CLI command shown inline in a highlighted code block

Changelog and Handoff updated

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

packages/app/src/hooks/useArtboards.ts

@@ -9,6 +9,8 @@ export interface CanvasArtboard {
   width: number;
   height: number;
   renderUrl?: string;
+  /** Route path appended to renderUrl when rendering a specific screen, e.g. "/dashboard". */
+  route?: string;
 }
 
 
@@ -21,6 +23,7 @@ function toCanvasArtboard(ab: Artboard): CanvasArtboard | null {
   if (x === null || y === null || width === null || height === null) return null;
   const base: CanvasArtboard = { id: ab.id, label: ab.name, x, y, width, height };
   if (typeof meta['renderUrl'] === 'string') base.renderUrl = meta['renderUrl'];
+  if (typeof meta['route'] === 'string' && meta['route']) base.route = meta['route'];
   return base;
 }