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/mcp-server/src/drawing-guide.ts

@@ -52,6 +52,8 @@ Containers and swimlanes: children use the container id as parent and coordinate
 
 ${indent(SWIMLANE_EXAMPLE)}
 
+Large diagrams: if the diagram will not fit one call, send the first cells with create_new_diagram and add the rest with edit_diagram add operations (one per cell, same id rules). A call rejected because the XML ends inside an unfinished cell drew nothing: send all of its cells again, in smaller parts if needed.
+
 ## Layout
 - Keep every element of a page within x 0 to 800 and y 0 to 600, so the whole diagram fits one view without a page break.
 - Containers (for example AWS cloud boxes) are at most 700 pixels wide and 550 pixels tall.
@@ -92,6 +94,9 @@ ${indent(STYLE_CLASS_EXAMPLE)}
 - Edges: endArrow=block, open or none; startArrow=classic; curved=1; dashed=1; edgeStyle=orthogonalEdgeStyle
 - Text: fontSize=14, fontStyle=1 (bold), align=center or right
 - Animated connectors: add flowAnimation=1 to the edge style.
+- Edge options: dashed=1; strokeWidth=2; edgeStyle=elbowEdgeStyle;elbow=vertical (or horizontal); jumpStyle=arc;jumpSize=10 where edges cross.
+
+For tables, layers or groups, call get_drawing_guide with topic=tables, layers or groups.
 
 ## Minimal style
 When the user asks for a minimal, plain, black-and-white or unstyled diagram, use these rules instead of the styles above: