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>

web/style.css

@@ -158,6 +158,7 @@ body {
 button, input, select { font: inherit; color: inherit; }
 button { cursor: pointer; }
 svg { fill: none; stroke: currentColor; stroke-width: 2; stroke-linecap: round; stroke-linejoin: round; }
+svg.pi { fill: currentColor; stroke: none; } /* Papicons: filled shapes, no outline */
 .muted { color: var(--muted); }
 .small { font-size: 12.5px; }
 [hidden] { display: none !important; }