Web UI write support (create issues, comment, review)

[LiranCohen]

Summary

The web UI (dwn-git web) is currently read-only. Add write capabilities so users can create issues, post comments, submit patches, and perform reviews directly from the browser.

Context

The web UI (PLAN.md Section 8, src/web/) renders DWN records as HTML pages. All mutations currently require the CLI or the GitHub API shim. Adding forms and POST handlers to the web UI would make it a self-contained forge experience.

Scope

• Create issue form (POST /:did/issues/new)
• Issue comment form (POST /:did/issues/:number/comment)
• Create patch form (POST /:did/patches/new)
• Patch review form (POST /:did/patches/:number/review)
• Edit wiki page form (POST /:did/wiki/:slug/edit)

Design considerations

• Authentication: reuse the DWN_GIT_API_TOKEN Bearer token approach, or session-based auth
• CSRF protection for form submissions
• Keep the "no client-side JS" philosophy, or introduce minimal JS for UX
• Consider whether write operations should go through handleShimRequest or directly to DWN