v0.4.0 — Config UI redesign & expanded project preferences

This release rebuilds contextzip config --ui and roughly doubles
what's configurable per project. No breaking changes — existing
.contextzip/config.json files keep working as-is; new fields default
to their prior hardcoded behavior when absent.

Config UI redesign

contextzip config --ui is now five tabs instead of one pane:
Files, AI selection, Workspace, Advanced, and a live
Raw JSON preview. The file tree and pattern list now render as a
diff — a solid gutter and +/- marks on every row — instead of
checkboxes. Still a single dependency-free HTML file: no build step,
no Node.js, no network calls from the page itself.

New project preferences

All editable from the UI or by hand in .contextzip/config.json:

• ai.prompt_template — house conventions prepended to every generated
  prompt.txt.
• limits.max_file_size_mb — per-project large-file threshold (was a
  fixed 1 MB).
• applied_zip_retention — how many past apply-zip archives to keep
  before pruning (was hardcoded to 1).
• webui.auto_open / webui.port — skip auto-launching a browser tab
  and/or pin the UI to a fixed local port.

Full changelog: see CHANGELOG.md