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

README.md

@@ -94,6 +94,7 @@ With a 4-hour cadence Pete will post at most 6 stories/day, so set `posting.dail
 | `-config <path>` | Path to config file (default: `config.toml`) |
 | `-seed` | Ingest all current feed items as seen without posting, then exit |
 | `-test` | Post one story to verify the full pipeline, then exit |
+| `-local` | Web/RSS-only mode: poll feeds and serve the web UI on `web.listen_addr`. Skips Matrix login and posting — useful for local testing |
 
 ## Docker