Build out the forge web UI

Build out the forge web UI

Turn the web surface into a compact, server-rendered forge with HTMX as the
progressive dynamic layer.

- Split templates and assets into embedded files; add a tokenised dark theme,
  Lucide icons, and vendored HTMX.
- Add the repository Overview page and a separate chronological Activity feed
  with type filters and HTMX fragment updates.
- Finish the issue and change workflows: create pages, comments, close/reopen,
  labels and assignees, approval, discussion, line-anchored review comments,
  and HTMX list filters.
- Add syntax-highlighted blobs, contributor/statistics derivation, branch-aware
  code browsing, and a repository Runs list.
- Add a loopback-only development login for local write testing, with explicit
  configuration and listener checks.
- Extend tests for the new pages, fragments, labels, assignees, anchors, and
  development login.