fix(market): back off failed market fetches instead of retrying every frame

The Market tab calls the exchange-list, price-history and candle
fetches every frame, so a failed fetch (offline, rate-limited, a
venue's API down) was retried on the very next frame, ~20 requests a
second.

- util::FetchBackoff: 30 s doubling to a 10 min cap, +/-20% jitter.
  The exchange list, the price history (a partial result counts as a
  failure) and the per-pair candles each back off on failure.
- Candle backoff is per pair. Switching to another pair still loads at
  once, a failed refresh keeps the on-screen pair's last candles, and
  switching back to a pair that is backing off shows the aggregate
  chart rather than the previous pair's candles.
- The Market tab's refresh button retries at once (at most every 5 s).
  A price fetch (60 s timer, no backoff) that succeeds after failing
  means the network is back, so it lifts all three backoffs.
- A partial price history no longer marks the chart loaded: with no
  fetch timestamp, the 30-minute reload gate measured system uptime
  and blocked retries for the first 30 minutes after boot.
- The exchange list goes through httpGetString (TLS verified, size
  capped) with short timeouts (5 s connect, 12 s total, as it runs on
  the shared RPC worker) instead of its own curl code. A failed fetch
  is retried; it used to be tried only once per session.

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

src/ui/windows/market_tab.cpp

@@ -1681,7 +1681,7 @@ static void mktDrawPriceChart(const MktCtx& cx)
                     refreshHov ? OnSurface() : OnSurfaceMedium(), ICON_MD_REFRESH);
         ImGui::SetCursorScreenPos(rbMin);
         if (ImGui::InvisibleButton("##RefreshMarket", ImVec2(pillH, pillH))) {
-            app->refreshMarketData();
+            app->retryMarketDataNow();   // also lifts a failure backoff, so a manual retry runs now
         }
         if (ImGui::IsItemHovered()) material::Tooltip("%s", TR("market_refresh_price"));