Build 75: no pairing by default; price back over the mixnet

- Pairing defaults to Off on first run: no conversion is shown anywhere and no
  price request leaves the device until the user opts into a pairing. Off is a
  picker option and the choice persists.
- The rate fetch goes back over the Nym mixnet (reverting Build 71's clearnet
  route) — it's opt-in and infrequent, so it stays private and doesn't get in
  the way of slatepacks. CoinGecko's free-tier rate limit (shared exit IP) rules
  out live/1s polling, so it keeps the slow cache.
- Small note on the Pairing page that rates fetch over the mixnet.

src/gui/views/goblin/mod.rs

@@ -2226,6 +2226,15 @@ impl GoblinWalletView {
 						}
 					}
 				});
+				ui.add_space(10.0);
+				ui.label(
+					RichText::new(
+						"Rates fetch over the Nym mixnet, only while a pairing is on — \
+						 off means no rate request leaves your device.",
+					)
+					.font(FontId::new(12.0, fonts::regular()))
+					.color(t.text_dim),
+				);
 				ui.add_space(16.0);
 			});
 	}

src/http/price.rs

@@ -12,15 +12,18 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//! GRIN price preview, cached per currency. The rate is non-sensitive metadata
-//! (no payment info), so — like the update check, and like upstream Grim — it is
-//! fetched direct over HTTPS rather than waiting on the mixnet, keeping the
-//! preview prompt. Payments, relays and identity stay mixnet-only.
+//! GRIN price preview, cached per currency. Off by default (no pairing → no
+//! fetch); only once the user opts into a pairing is the rate fetched — and it
+//! goes over the Nym mixnet like everything else, never the clear net. The rate
+//! barely moves and CoinGecko's free tier rate-limits frequent polling (the
+//! shared exit IP all the more), so it refreshes on a slow cache, not live.
 
 use lazy_static::lazy_static;
 use parking_lot::RwLock;
 use std::collections::{HashMap, HashSet};
-use std::time::{Duration, SystemTime, UNIX_EPOCH};
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use crate::nym;
 
 /// Cache refresh interval (seconds).
 const REFRESH_SECS: i64 = 300;
@@ -91,20 +94,16 @@ fn trigger_refresh(vs: String) {
 	});
 }
 
-/// Fetch the GRIN/`vs` rate from CoinGecko, direct over HTTPS.
+/// Fetch the GRIN/`vs` rate from CoinGecko over the Nym mixnet.
 async fn fetch_rate(vs: &str) -> Option<f64> {
 	let url = format!(
 		"https://api.coingecko.com/api/v3/simple/price?ids=grin&vs_currencies={}",
 		vs
 	);
 	// CoinGecko rejects requests without a User-Agent (403). A static,
-	// non-identifying UA.
-	let client = reqwest::Client::builder()
-		.user_agent("goblin-wallet")
-		.timeout(Duration::from_secs(20))
-		.build()
-		.ok()?;
-	let body = client.get(&url).send().await.ok()?.text().await.ok()?;
+	// non-identifying UA is fine over the mixnet.
+	let headers = vec![("User-Agent".to_string(), "goblin-wallet".to_string())];
+	let body = nym::http_request("GET", url, None, headers).await?;
 	let parsed: Option<f64> = serde_json::from_str::<serde_json::Value>(&body)
 		.ok()
 		.and_then(|doc| doc.get("grin")?.get(vs)?.as_f64());

src/settings/config.rs

@@ -458,9 +458,12 @@ impl AppConfig {
 				return p;
 			}
 		}
+		// No pairing chosen yet → off by default: no conversion is shown anywhere
+		// and no price is fetched until the user opts into a pairing. (A legacy
+		// `fiat_preview = true` still defaults to USD for existing users.)
 		match r_config.fiat_preview {
-			Some(false) => Pairing::Off,
-			_ => Pairing::Usd,
+			Some(true) => Pairing::Usd,
+			_ => Pairing::Off,
 		}
 	}