Add browser tab icon (src/app/icon.svg)

Volt green rounded square with a white sigma (power balance), so the
three tools (Leistungsbilanz, Netzberechnung, Verteilerplan) can be told
apart in the browser tab. Netzberechnung uses petrol with a transformer
symbol, Verteilerplan copper with a single-line motif. Next.js links the
file automatically as <link rel="icon">; checked with next build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/icon.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32"><rect width="32" height="32" rx="7" fill="#237a47"/><path d="M22.5 8.5h-13l7 7.5-7 7.5h13" fill="none" stroke="#ffffff" stroke-width="3.2" stroke-linecap="round" stroke-linejoin="round"/></svg>