feat: test signal via avatar/indicator tap on main screen

Tapping user avatar or network indicator sends a test signal with
geo data. Backend formats it as "Тест от username" (🧪) instead of
"Сигнал" (🚨). Only active after login on main screen.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/main.py

@@ -229,11 +229,18 @@ async def signal(
         if geo
         else "Гео нету"
     )
-    text = (
-        f"🚨 Сигнал от {user_name}\n"
-        f"⏰ {ts.strftime('%H:%M:%S')} UTC\n"
-        f"{geo_info}"
-    )
+    if body.is_test:
+        text = (
+            f"🧪 Тест от {user_name}\n"
+            f"⏰ {ts.strftime('%H:%M:%S')} UTC\n"
+            f"{geo_info}"
+        )
+    else:
+        text = (
+            f"🚨 Сигнал от {user_name}\n"
+            f"⏰ {ts.strftime('%H:%M:%S')} UTC\n"
+            f"{geo_info}"
+        )
     asyncio.create_task(telegram.send_message(text))
 
     return SignalResponse(status="ok", signal_id=signal_id)