fix(slice-011): code correctness and clarity improvements

[fredericomozzato]

Summary

• Fix silently dropped io.ReadAll errors in internal/api/coingecko.go — both FetchMarkets and FetchPrices now propagate read errors from non-2xx response bodies instead of ignoring them
• Replace magic numbers with named constants in internal/ui/markets.go — coinFetchLimit (100), refreshInterval (60s), grouped alongside existing staleThreshold
• Add documentation comments in cmd/crypto-tracker/main.go — cancellation safety assumption for shutdown and DB lifecycle ownership
• Expand comment explaining the background message broadcast pattern in internal/ui/app.go

Test Plan

• make check passes (fmt, lint, test, vuln)
• Error messages from CoinGecko client still include status code and body when available
• No change in runtime behaviour — constants match previous literal values