Local mode, Makefile, image fixes, Pete avatar

- Add -local flag: web/RSS-only mode that skips Matrix login and posting,
  so the web UI can be exercised against live feeds without credentials
- Add Makefile (build/local/seed/test/clean) that handles Tailwind +
  go build in one shot
- Fix Guardian thumbnails: NormalizeImageURL was rewriting width=1200
  onto signed i.guim.co.uk URLs, invalidating the s= signature and
  returning 401. Leave signed URLs alone and pick the widest
  media:content variant up front instead
- Use pete.avif as the header logo, favicon, and footer mark; drop the
  unused leaf.svg

internal/web/static/img/leaf.svg

@@ -1,4 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
-  <path d="M48 8C28 8 12 22 12 42c0 6 2 12 6 16 4-12 14-22 26-26-10 8-16 18-18 30 16 0 30-14 30-34 0-8-4-16-8-20z" fill="#6db73c" stroke="#3f6e1f" stroke-width="3" stroke-linejoin="round"/>
-  <path d="M22 52c8-12 18-20 30-26" stroke="#3f6e1f" stroke-width="2" fill="none" stroke-linecap="round"/>
-</svg>