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.

.github/workflows/electron-release.yml

@@ -34,15 +34,7 @@ jobs:
           node-version: 24
           cache: "npm"
 
-      - name: Download draw.io static files for offline use
-        run: |
-          rm -rf public/drawio
-          git clone --depth 1 https://github.com/jgraph/drawio.git /tmp/drawio
-          mkdir -p public/drawio
-          cp -r /tmp/drawio/src/main/webapp/* public/drawio/
-          rm -rf public/drawio/WEB-INF
-          rm -rf public/drawio/META-INF
-
+      # npm run build downloads draw.io first (prebuild)
       - name: Install dependencies
         run: npm install
 
@@ -66,16 +58,7 @@ jobs:
           node-version: 24
           cache: "npm"
 
-      - name: Download draw.io static files for offline use
-        shell: bash
-        run: |
-          rm -rf public/drawio
-          git clone --depth 1 https://github.com/jgraph/drawio.git /tmp/drawio
-          mkdir -p public/drawio
-          cp -r /tmp/drawio/src/main/webapp/* public/drawio/
-          rm -rf public/drawio/WEB-INF
-          rm -rf public/drawio/META-INF
-
+      # npm run build downloads draw.io first (prebuild)
       - name: Install dependencies
         run: npm install