Remove GRASS from crypto-to-USD conversion

Grass points are an internal reward system, not the GRASS token
traded on exchanges. Converting at token price ($0.29) wildly
overstates the value. Show points without USD conversion.

Author: GeiserX

app/exchange_rates.py

@@ -19,9 +19,11 @@
 
 # CoinGecko IDs for crypto tokens tracked by CashPilot collectors.
 # Map: our internal currency code -> CoinGecko coin id
+# Note: Grass *points* are NOT the GRASS token — they're an internal reward
+# that converts to tokens only during airdrops at unknown ratios, so we
+# intentionally do NOT map GRASS here.
 CRYPTO_IDS: dict[str, str] = {
     "MYST": "mysterium",
-    "GRASS": "grass",
 }
 
 CACHE_TTL = 900  # 15 minutes