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/models.py

@@ -25,6 +25,7 @@ class SignalRequest(BaseModel):
     user_id: Optional[str] = None  # UUID for legacy api_key auth; omit for JWT auth
     timestamp: int = Field(..., gt=0)
     geo: Optional[GeoData] = None
+    is_test: bool = False
 
 
 class SignalResponse(BaseModel):