Adds File > Export as PDF... menu item (Cmd+Shift+E) and an overflow
menu entry that renders the active markdown preview through a sandboxed
Electron BrowserWindow and writes it to disk via printToPDF.
- New main-side IPC handler (src/main/ipc/export.ts) and html-to-pdf
helper that loads a CSP-locked HTML document in a sandboxed, context-
isolated window with javascript enabled only for image-ready polling.
- Renderer helpers clone the rendered markdown subtree, inline all
computed styles through a curated allowlist, and ship the resulting
HTML fragment over IPC.
- Ref-counted listener registration so split-pane layouts install
exactly one IPC subscription and survive panel churn.
