UI: replace Lucide icons with Papicons

Swap the inline SVG icons of the web UI (top bar, view mode, time range,
export, close, empty state, back to top, settings tabs, save, delete,
lock) for their Papicons equivalents (MIT, Association Papillon).
Papicons are filled shapes: they carry class "pi", which sets
fill: currentColor and no stroke, so the stroked Lucide icons that stay
(zoom out, Docker badge) are unaffected. Add THIRD_PARTY_NOTICES.md with
the Papicons MIT notice and a pointer from both READMEs.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

README.md

@@ -671,3 +671,8 @@ To update one of them:
 With Docker Desktop (macOS/Windows), the source IP seen by the container for UDP packets is
 the Docker gateway. The `host` field still comes from the syslog header, which normally
 carries the sender's real name.
+
+## Third-party assets
+
+UI icons come from [Papicons](https://github.com/PapillonApp/papicons) (MIT) and, for a few of them,
+[Lucide](https://lucide.dev) (ISC). Licenses and notices: [THIRD_PARTY_NOTICES.md](THIRD_PARTY_NOTICES.md).