DOCX Mermaid diagrams fall back to code in macOS releases #59

Description

DOCX exports produced by an installed macOS release render Mermaid fenced blocks as source-code fallbacks instead of embedded diagram images.

This was found after the DOCX export implementation landed in #58.

Root cause

Release builds embed webui/dist, but Mermaid rendering currently searches for a standalone bundle only in development or sidecar locations such as webui/node_modules/mermaid/dist/mermaid.min.js and mermaid.min.js next to the executable. A normal macOS release contains neither, so findMermaidJS() fails and the DOCX exporter deliberately falls back to a code block.

Browser discovery can also fail when Chrome-family browsers are installed under ~/Applications, because only system /Applications paths and $PATH are checked.

Expected behavior

• Copy a standalone mermaid.min.js into webui/dist during the WebUI build.
• Include it in the existing embedded WebUI filesystem used by release binaries.
• Load the embedded Mermaid bundle for PDF and DOCX rendering rather than requiring node_modules or a sidecar file.
• Preserve development-path fallbacks where useful.
• Discover supported macOS browsers in both /Applications and ~/Applications.
• Add tests covering embedded Mermaid discovery and macOS browser candidate construction.

Actual behavior

The exporter logs docx: Mermaid render failed; using code fallback, commonly with mermaid.min.js not found, and writes the Mermaid definition as a highlighted code block.