refactor(market): resolve exchange/pair selection through one helper

The chart fetch and the Market tab both resolved the saved exchange and
pair names into the registry, with their own near-identical scans and
slightly different fallbacks. Keeping two copies risked the chart and
the chips disagreeing about which pair was selected.

- A single resolver maps the saved names to a registry index, matching
  the exchange name (any exchange when none is saved) then the pair
  display name, and falling back to the registry's first pair, the UI
  default. The chart fetch and the Market tab both call it.
- The Market tab now derives its selection indices from the saved names
  against the current registry every frame, so the indices stay correct
  when the live exchange list replaces the fallback; a steady-state fast
  path skips the scan once the indices already name the saved selection.

Tests: the resolver returns the right index for a matched exchange/pair,
an empty-exchange match, and the first-pair fallback on misses.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/data/exchange_info.h

@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include <cstddef>
 #include <string>
 #include <vector>
 
@@ -38,6 +39,25 @@ struct ExchangeInfo {
  */
 const std::vector<ExchangeInfo>& getExchangeRegistry();
 
+/**
+ * @brief Index of the selected exchange/pair in `reg`, resolved by saved NAME.
+ * Matches the exchange name (any exchange when selEx is empty) THEN the pair display name, like the chart
+ * fetch always has. Not found => (0,0), found=false (the registry's first pair - the app's default).
+ * Single source of truth for the Market tab chips and App::refreshExchangeChart so they cannot diverge.
+ */
+struct ExchangeSelection { int exchangeIdx = 0; int pairIdx = 0; bool found = false; };
+inline ExchangeSelection resolveExchangeSelection(const std::vector<ExchangeInfo>& reg,
+                                                  const std::string& selEx, const std::string& selPair)
+{
+    for (std::size_t ei = 0; ei < reg.size(); ++ei) {
+        if (!selEx.empty() && reg[ei].name != selEx) continue;
+        for (std::size_t pi = 0; pi < reg[ei].pairs.size(); ++pi)
+            if (reg[ei].pairs[pi].displayName == selPair)
+                return {static_cast<int>(ei), static_cast<int>(pi), true};
+    }
+    return {};
+}
+
 /**
  * @brief Parse a CoinGecko /coins/{id}/tickers JSON body into the exchange registry.
  *