fix: add 10MB body size limit to MCP HTTP endpoints

All three POST handlers (/api/state, /api/restore, /api/history-svg)
now use a shared readBody() helper that enforces a 10MB limit and
returns 413 if exceeded, preventing memory exhaustion from oversized
requests.

Bumps @next-ai-drawio/mcp-server to 0.1.19.

.github/workflows/electron-release.yml

@@ -37,7 +37,7 @@ jobs:
       - 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
+          git clone --depth 1 --branch v29.3.5 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
@@ -70,7 +70,7 @@ jobs:
         shell: bash
         run: |
           rm -rf public/drawio
-          git clone --depth 1 https://github.com/jgraph/drawio.git /tmp/drawio
+          git clone --depth 1 --branch v29.3.5 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