feat(mcp-server): load .drawio.svg, change summary, XML reference by topic, preview options, truncation message (#982)

* feat(mcp-server): load .drawio.svg (Editable SVG) files with load_diagram

* feat(mcp-server): summarize the user's manual changes per cell in stale rejections and get_diagram

* feat(mcp-server): XML reference for tables, layers and groups via get_drawing_guide topic

* feat(mcp-server): DRAWIO_LANG, DRAWIO_UI and DRAWIO_DARK for the preview page

Three host-config environment variables fix the language, theme and
dark mode of the draw.io editor embedded in the preview page. The new
drawio-themes module holds the theme list and draw.io's locale names
(zh-hant becomes zh-tw); drawioEmbedParams() in http-server.ts builds
the variable tail of the iframe query and getHtmlPage fills the new
{{DRAWIO_PARAMS}} placeholder. Without the variables the page keeps
sending dark=auto as before. Both READMEs document the variables.

* feat(mcp-server): report XML cut off inside a cell and explain drawing in parts

create_new_diagram and add_page now detect bare-cell XML that ends inside an
unfinished mxCell (XML comments stripped first) and return an error with the
last 300 characters of the input, asking the model to resend from that cell or
continue with edit_diagram add operations. isMxCellXmlComplete moves from
lib/utils.ts into packages/mcp-server/src/new-diagram.ts and is re-exported
from lib/utils.ts for the web app. The drawing guide gains a "Large diagrams"
paragraph and the INSTRUCTIONS edit_diagram line mentions drawing in parts.

* fix(mcp-server): review fixes for load .drawio.svg, change summary, XML reference, draw.io embed options, truncation message

* fix(mcp-server): Codex review fixes for .drawio.svg loading, change summary, references, embed options and truncation

- Truncation check: a closing tag such as </mxCell/> that the auto-fix
  repairs is no cut, and input without any cell keeps the validator's
  message
- Guide: a call rejected as cut off drew nothing, so all of its cells are
  sent again; the topic pointer is its own paragraph
- Change summary: decoded labels (no &nbsp; or merged words from <br>),
  a fast path for equal XML, and "the order of the cells changed" as
  the fallback
- DRAWIO_UI in any case, DRAWIO_LANG=zh-Hans maps to draw.io's zh
- load_diagram and export_diagram describe which files load again

packages/claude-plugin/README.md

@@ -94,7 +94,7 @@ Recreate the whiteboard photo at ~/Desktop/sketch.jpg as a clean draw.io diagram
 | `get_drawing_guide` | Return the drawing rules again |
 | `get_shape_library` | Return the shapes and icon styles of a library such as `aws4` |
 | `create_new_diagram` | Create a new diagram from XML |
-| `load_diagram` | Load a `.drawio` file from disk |
+| `load_diagram` | Load a `.drawio` or `.drawio.svg` file from disk |
 | `edit_diagram` | Edit diagram by ID-based operations; all or nothing |
 | `get_diagram` | Get the current diagram XML |
 | `screenshot_diagram` | Return a PNG of a page so Claude can check the result |
@@ -120,6 +120,9 @@ Claude Code <--stdio--> MCP Server <--http--> Browser (draw.io)
 | `PORT` | `6002` | Port for the embedded HTTP server |
 | `DRAWIO_BASE_URL` | `https://embed.diagrams.net` | Base URL for draw.io (for self-hosted deployments) |
 | `DRAWIO_DATA_DIR` | `~/.next-ai-drawio` | Folder for auto-saved diagrams and your `instructions.md`; `off` turns auto-save off (`instructions.md` is then read from the default folder) |
+| `DRAWIO_LANG` | unset | Language of the draw.io editor, such as `en`, `zh`, `zh-tw`, `ja` or `de`. Unset, draw.io chooses (the browser language on `embed.diagrams.net`, English on a self-hosted draw.io) and the user can change it under **Extras > Language** |
+| `DRAWIO_UI` | unset | draw.io theme: `kennedy`, `atlas`, `dark`, `min`, `sketch` or `simple`. Unset, the user picks one under **Extras > Theme** |
+| `DRAWIO_DARK` | `auto` | Dark mode of the draw.io editor: `auto` follows the system, `1` forces dark, `0` forces light |
 | `DEBUG` | unset | Set to `true` to log debug messages |
 
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