Add --watch mode, Unix socket live-push, and stdin support

[Halicea]

Summary

• Watch mode (--watch / -w): Monitors the open file with GFileMonitor and
  Switching files via the toolbar transparently moves the monitor to the new file.
• Unix socket live-push (--socket): When launched with --socket, ViewMD
  creates /tmp/viewmd-<pid>.sock and prints VIEWMD_SOCKET=<path> to stdout. Editor
  plugins can connect, push raw Markdown, and optionally prefix the payload with
  CURSOR:<line>\n to scroll the preview to the current edit position — no disk write
  required.
• Stdin support (-): Passing - as the filename reads content from stdin
  (cat file.md | viewmd -).

Test plan

• viewmd --watch file.md — edit and save, confirm auto-reload
• Same file via toolbar Reload while --watch is active — monitor must not
  restart
• Open a different file via toolbar while --watch is active — monitor follows
• viewmd --socket file.md, then echo "# Hello" | nc -U $VIEWMD_SOCKET —
  content updates live
• Socket with cursor: { printf 'CURSOR:5\n'; cat file.md; } | nc -U $VIEWMD_SOCKET — scrolls to line 5
• cat file.md | viewmd - — renders correctly
• viewmd - from a terminal (no pipe) — prints error, does not hang
• Normal viewmd file.md launch — no socket file created, no extra stdout
  output
• Quit while --watch or --socket is active — no crash, socket file cleaned
  up

🤖 Generated with Claude Code

[rabfulton]

Some good ideas here, I like the watch option! Two minor issure for your consideration:

1. Your line scrolling does anot account for line wrapping so will not always scroll to the correct place in a document. There is already some scrolling functions in the codebase that tackle this issue when handling internal links gtk_text_view_scroll_to_mark().

2. It would be preferrable if you can make the socket input feature make nonblocking so that it can never hang the UI thread.

Thanks.