feat(go/fio): debug logging via slog at LOG_LEVEL=DEBUG

Wires slog.SetDefault to honour LOG_LEVEL in all CLI commands and adds
debug logs on the Fio fetch path so a silent "fetched 0 transaction(s)"
can be diagnosed without code changes:

- fio.New: which client variant (api/transparent) was selected
- apiClient: GET URL (token redacted as ****), HTTP status, body bytes,
  parsed transaction count
- transparentClient: GET URL, HTTP status, body bytes, plus parser
  stats (raw rows from second table, kept, dropped_bad_date,
  dropped_nonpositive_amount)

Also suppresses the --print-fio-table block when zero transactions were
fetched, so the bare header no longer prints under that condition.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/services/banksync/sync.go

@@ -112,8 +112,8 @@ func SyncToSheets(
 		})
 	}
 
-	// 4b. Optional debug table (dry-run only).
-	if opts.DryRun && opts.PrintFioTable {
+	// 4b. Optional debug table (dry-run only; suppress when nothing was fetched).
+	if opts.DryRun && opts.PrintFioTable && len(txns) > 0 {
 		printFioTable(os.Stdout, txns, syncIDs, existingIDs)
 	}