feat(ui): canvas-first redesign with a self-hosted draw.io

The canvas now fills the window and the chat floats on its right. draw.io is
served from our own origin (public/drawio, downloaded at build time and
checked against the release's SHA-256), so the app can drive the editor
directly:

- every AI change becomes a version card with a thumbnail; compare, restore
  or undo it, and Ctrl+Z on the canvas takes an AI change back in one step
- shapes the AI just changed are highlighted on the canvas
- selected shapes appear as a chip and are sent to the model
- the app's own canvas toolbar, shape library and page tabs replace
  draw.io's chrome
- new start screen, session rename and switcher, open .drawio files
- settings split into Models & keys, Appearance, Advanced and About
- phones switch between canvas and chat
- dark mode keeps the diagram on a white sheet and no longer flashes

With an external draw.io (NEXT_PUBLIC_DRAWIO_BASE_URL) the app falls back to
draw.io's own toolbar and the features above that need same origin are off.

The chat logic moved from the 1500-line chat panel into a chat engine plus
small UI components; zustand holds canvas, versions, settings and UI state.

package.json

@@ -5,7 +5,9 @@
     "private": true,
     "main": "dist-electron/main/index.js",
     "scripts": {
+        "predev": "node scripts/fetch-drawio.mjs",
         "dev": "next dev --turbopack --port 6002",
+        "prebuild": "node scripts/fetch-drawio.mjs",
         "build": "next build",
         "start": "next start --port 6001",
         "lint": "biome lint .",
@@ -82,10 +84,8 @@
         "prism-react-renderer": "^2.4.1",
         "react": "^19.1.2",
         "react-dom": "^19.1.2",
-        "react-drawio": "^1.0.3",
         "react-icons": "^5.5.0",
         "react-markdown": "^10.1.0",
-        "react-resizable-panels": "^3.0.6",
         "remark-gfm": "^4.0.1",
         "saxes": "^6.0.0",
         "server-only": "^0.0.1",
@@ -94,7 +94,8 @@
         "tailwindcss-animate": "^1.0.7",
         "turndown": "^7.2.0",
         "unpdf": "^1.4.0",
-        "zod": "^4.1.12"
+        "zod": "^4.1.12",
+        "zustand": "^5.0.15"
     },
     "optionalDependencies": {
         "@tailwindcss/oxide-linux-x64-gnu": "^4.1.18",