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

Makefile

@@ -0,0 +1,25 @@
+.PHONY: build css go-build local seed test clean
+
+build: css go-build
+
+css: node_modules
+	npm run build:css
+
+node_modules: package.json package-lock.json
+	npm install
+	@touch node_modules
+
+go-build:
+	go build -tags goolm -o pete .
+
+local: build
+	./pete -local
+
+seed: build
+	./pete -seed
+
+test:
+	go test ./...
+
+clean:
+	rm -f pete internal/web/static/css/output.css