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/util/http_download.cpp

@@ -49,7 +49,8 @@ int xferCb(void* clientp, curl_off_t dltotal, curl_off_t dlnow, curl_off_t, curl
 
 } // namespace
 
-std::string httpGetString(const std::string& url, const char* logTag)
+std::string httpGetString(const std::string& url, const char* logTag,
+                          long connectTimeoutSec, long timeoutSec)
 {
     CURL* curl = curl_easy_init();
     if (!curl) return {};
@@ -59,8 +60,8 @@ std::string httpGetString(const std::string& url, const char* logTag)
     curl_easy_setopt(curl, CURLOPT_WRITEDATA, &result);
     curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
     curl_easy_setopt(curl, CURLOPT_USERAGENT, "ObsidianDragon/1.0");
-    curl_easy_setopt(curl, CURLOPT_TIMEOUT, 30L);
-    curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 15L);
+    curl_easy_setopt(curl, CURLOPT_TIMEOUT, timeoutSec);
+    curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, connectTimeoutSec);
     curl_easy_setopt(curl, CURLOPT_MAXFILESIZE_LARGE, static_cast<curl_off_t>(kMaxMetadataBytes));  // reject oversized metadata
     curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 1L);
     curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 2L);